In response to criticism of latent fingerprint evidence from a variety of authoritative extra-legal inquiries and reports, this essay describes the first iteration of a guide designed to assist with the reporting and interpretation of latent fingerprint evidence. Sensitive to the recommendations of these reports, we have endeavoured to incorporate emerging empirical evidence about the matching performance of fingerprint examiners (i.e. indicative error rates) into their testimony. We outline a way of approaching fingerprint evidence that provides a more accurate—in the sense of empirically and theoretically justified—indication of the value of fingerprint evidence than existing practice. It is an approach that could be introduced immediately. The proposal is intended to help non-experts understand the value of the evidence and improve its presentation and assessment in criminal investigations and proceedings. This first iteration accommodates existing empirical evidence and draws attention to the gap between the declaration of a match and positive identification (or individualization). Represented in this way, fingerprint evidence will be more consistent with its known value as well as the aims and conduct of the accusatorial trial.
1. Reforming the presentation of comparison evidence
Fingerprint examiners have been active in investigations and presented ‘identification’ evidence in criminal courts for more than a century.1 Notwithstanding increasing automation, examiners continue to play a central role in the comparison of prints, the interpretation of prints, and in attributing significance to apparent matches. When confronted with an unknown print, usually a part (or fragment) of a fingermark recovered from a crime scene (known as a ‘latent’), it is the examiner who decides if the latent print provides sufficient information to interpret and, if so, whether it matches a known (i.e. reference) print.2 Where the examiner is satisfied about the sufficiency of the print and declares a ‘match’, this is conventionally understood by examiners, and represented to others, as positive identification of the person who supplied the reference print to the exclusion of all other persons.3
Remarkably, given the interpretive (i.e. subjective) dimensions of comparison and the considerable gap between declaring a match and positive identification (so-called individualization),4 there have been few scientific investigations of the human capacity to correctly match fingerprints, let alone attach significance to apparent similarities.5 Nevertheless, for more than a hundred years, and in the absence of experimental support, fingerprint examiners have claimed that fingerprint evidence is basically infallible.6 These assertions are typically justified by reference to training and experience (and the use of a method such as ACE-V: Analysis, Comparison, Evaluation and Verification),7 assumptions about the uniqueness of fingerprints, along with legal acceptance and the effectiveness of fingerprint evidence in securing confessions and convictions.8 In recent decades, however, commentators have questioned uniqueness (and its significance) and dismissed claims about error-free, positive identification as scientifically implausible. In recent years, these doubts have materialized in notorious mistakes, and scholarly criticisms endorsed in independent inquiries and reports—discussed in Section 2.9
This article presents a first iteration of what we envisage could be an evolving response to the vexed issue of the reporting (or expression) of forensic comparison evidence.10 Conceived as a practical aid to assist with the presentation and interpretation of forensic science evidence, the guide to interpreting forensic science testimony (or Guide) is intended to embody the current state of relevant scientific research in relation to a particular technique (or set of techniques). This empirically predicated guide is designed to assist with the evaluation of evidence by highlighting areas of demonstrated expertise and incorporating an indicative error rate to assist with the assessment of expert opinion.11 Using the example of fingerprints, it would enable a fingerprint examiner to express an opinion about whether two prints match (or do not match) against the backdrop of an empirically derived error rate and other indicators of expertise and its limitations. Introducing an error rate into the provision of comparison evidence assists with the evaluation of opinions and in delimiting the scope of expertise.12
2. Background to the Guide: evidence, expertise, error and advice
2.1 Authoritative reports and recommendations
In a landmark report issued in 2009, a committee of the National Research Council (NRC) of the US National Academy of Sciences (NAS) drew attention to questionable practices and the lack of research in many areas of forensic science. The committee was surprised to discover that many forensic science disciplines are typically not supported by scientific research and that analysts are not necessarily bound by experimentally derived standards to ensure the evidence offered in courts is valid and reliable.13 In confronting language, the report itself states:
Often in criminal prosecutions and civil litigation, forensic evidence is offered to support conclusions about “individualization” (sometimes referred to as “matching” a specimen to a particular individual or other source) or about classification of the source of the specimen into one of several categories. With the exception of nuclear DNA analysis, however, no forensic method has been rigorously shown to have the capacity to consistently, and with a high degree of certainty, demonstrate a connection between evidence and a specific individual or source.14
In relation to latent fingerprint comparison, the NRC report explicitly challenged the dominant method—that is, ACE-V.15
ACE-V provides a broadly stated framework for conducting friction ridge analyses. However, this framework is not specific enough to qualify as a validated method for this type of analysis. ACE-V does not guard against bias; is too broad to ensure repeatability and transparency; and does not guarantee that two analysts following it will obtain the same results. For these reasons, merely following the steps of ACE-V does not imply that one is proceeding in a scientific manner or producing reliable results.16
The Committee also confronted and rejected the idea that fingerprint comparisons are free from error.
Errors can occur with any judgment-based method, especially when the factors that lead to the ultimate judgment are not documented. Some in the latent print community argue that the method itself, if followed correctly (i.e., by well-trained examiners properly using the method), has a zero error rate. Clearly, this assertion is unrealistic, and, moreover, it does not lead to a process of method improvement. The method, and the performance of those who use it, are inextricably linked, and both involve multiple sources of error (e.g., errors in executing the process steps, as well as errors in human judgment).17
The NRC report highlighted the absence of experiments on human expertise in forensic comparison (or pattern matching): ‘The simple reality is that the interpretation of forensic evidence is not always based on scientific studies to determine its validity.’ Going further, it concluded that: ‘[t]his is a serious problem’.18 The Committee recommended that US Congress fund basic research to help the forensic community strengthen their research foundations, develop valid and reliable measures of performance, and establish evidence-based standards for analyzing and reporting results. The Committee placed emphasis on addressing the limited research base, determining error rates, as well as understanding and reducing the effects of bias and human error.19
The NRC is not alone in its expressed concerns about forensic science used for the purposes of identification. Subsequent to the NRC report, two prominent inquiries in the US and the UK have released reports focused directly on fingerprint evidence. One report was produced by the Expert Working Group on Human Factors in Latent Print Analysis (EWGHF)—a large multidisciplinary collective jointly sponsored by the United States National Institute of Standards and Technology (NIST) and National Institute of Justice (NIJ)—Latent Print Examination and Human Factors.20 The other emerged from an inquiry conducted by Lord Campbell into problems with fingerprint evidence following the controversial McKie case in Scotland—The Fingerprint Inquiry (SFI).21 These reports, from jurisdictions generally regarded as leading forensic science providers, are again surprisingly critical in their responses to widely accepted identification practices.
Convened in 2008, in the shadow of the NRC inquiry, the EWGHF was tasked with undertaking a scientific assessment of the effects of human factors on latent print analysis.22 Specifically, the group was directed to evaluate current practices, to explain how human factors contribute to errors, and to offer guidance and recommendations. One way to obtain an impression of the Report’s main thrust is to consider its recommendations, particularly those relating to the comparison of fingerprints and the expression of results. Relevant recommendations include:
Recommendation 3.3: Procedures should be implemented to protect examiners from exposure to extraneous (domain-irrelevant) information in a case.
Recommendation 3.7: Because empirical evidence and statistical reasoning do not support a source attribution to the exclusion of all other individuals in the world, latent print examiners should not report or testify, directly or by implication, to a source attribution to the exclusion of all others in the world.
Recommendation 3.9: The federal government should support a research program that aims to:
Develop measures and metrics relevant to the analysis of latent prints;
Use such metrics to assess the reproducibility, reliability, and validity of various interpretive stages of latent print analysis; and
Identify key factors related to variations in performance of latent print examiners during the interpretation process.
Recommendation 6.3: A testifying expert should be familiar with the literature related to error rates. A testifying expert should be prepared to describe the steps taken in the examination process to reduce the risk of observational and judgmental error. The expert should not state that errors are inherently impossible or that a method inherently has a zero error rate.
Recommendation 9.1: Management should foster a culture in which it is understood that some human error is inevitable and that openness about errors leads to improvements in practice.
Recommendation 9.5: The latent print community should develop and implement a comprehensive testing program that includes competency testing, certification testing, and proficiency testing.23
The Scottish inquiry into the controversy surrounding the mistaken attribution of a latent print collected from a crime scene to Shirley McKie (a police officer) also generated a large, though perhaps less systemically oriented, report along with a series of recommendations.24 Under the heading ‘The subjective nature of fingerprint evidence’, recommendations from the SFI included:
Recommendation 1: Fingerprint evidence should be recognised as opinion evidence, not fact, and those involved in the criminal justice system need to assess it as such on its merits.25
Recommendation 2: Examiners should receive training which emphasises that their findings are based on their personal opinion; and that this opinion is influenced by the quality of the materials that are examined, their ability to observe detail in mark and print reliably, the subjective interpretation of observed characteristics, the cogency of explanations for any differences and the subjective view of ‘sufficiency’.26
Recommendation 3: Examiners should discontinue reporting conclusions on identification or exclusion with a claim to 100% certainty or on any other basis suggesting that fingerprint evidence is infallible.27
Beneath the heading ‘Fingerprint methodology,’ recommendations were particularly concerned with contextual bias:
Recommendation 6: The SPSA [Scottish Police Services Authority] should review its procedures to reduce the risk of contextual bias.28
From these independent inquiries, supported by a range of scientific studies and pre-existing scholarly critiques, several consensus themes emerge that are consistent with this proposal.29 Most prominent are: confirmation about the lack of scientific support for contemporary fingerprint comparison practice and underlying assumptions (NRC Rec. 3, LPEHF 3.9 and SFI 2, respectively); the rejection of claims about an infallible method and a zero error rate (NRC p.142, LPEHF 6.3 and SFI 3); and, concern about equating a declared ‘match’ with positive identification (NRC 3, LPEHF 3.7 and SFI 3).30 The reports place considerable emphasis on the need for research, standards derived from research (NRC 1, 7, 8, LPEHF 3.4, 3.6, 3.8, 3.9, 8.1), the need to attend to a range of potential biases, and the possibility of shielding analysts from some kinds of information (NRC 5, LPEHF 3.3 and SFI 6, 7, 8). The reports also recognize the need to present opinions derived from fingerprints in a manner that embodies their value and is simultaneously comprehensible to the tribunal of fact (NRC Rec. 2, LPEHF 5.1, SFI 64). The LPEHF Report (Rec. 4.3, 6.3, 9.1, and 9.2) directs attention to the need for examiners to be familiar with error rates, probabilities and statistics and the SFI (Rec. 82, 83) advocates the development of probabilities.
For the average reader—whether lawyer, judge or potential juror—all of this might come as something of a surprise. For, notwithstanding long reliance on fingerprint evidence, relatively little is known about the performance of fingerprint examiners or the value of their opinions. Contemporary investigative practices and reporting appear to fall well short of the recommendations and advice outlined in the independent reports. Concerns expressed by the NRC, EWGHF, and Lord Campbell, along with several notorious cases of fingerprint misattribution, raise serious (and as yet unresolved) questions about the forensic use of fingerprint evidence.31 There is, however, an indisputable need to reform the way fingerprint examiners work as well as the manner in which they express their opinions.
Currently, there is a dearth of research. The necessary studies are often beyond the capabilities and competence of fingerprint examiners (and yet to be undertaken, or completed). Understandably, the training of fingerprint examiners is primarily oriented toward comparing fingerprints. Most do not have the methodological skills, funding, time, infrastructure, or experience with research techniques to mount scientific studies of human performance. Moreover, few fingerprint examiners, lawyers, or judges have the time, resources or expertise to track and evaluate extant studies, inquiries and reports, or respond to research as it emerges.32 Consequently, changes to practices and reporting will require the ongoing assistance of research scientists.33 Research into expertise and complex sociotechnical systems is the domain of cognitive science and human factors. Researchers in these areas already have the infrastructure in place to conduct the requisite studies, and are well positioned to work with examiners to strengthen the field.
2.2 Emerging studies
Scholarly criticisms and recent inquiries have already spawned a range of studies. The first studies focused on consistency and bias in expert decisions, but it is difficult to glean indicators on human matching performance from them.34 Most of the research is currently in progress, though three studies have recently been published. In a controlled fingerprint matching experiment, Tangen et al. found that examiners incorrectly declared 0.68% of similar non-matching prints as ‘matching’ (false positive errors)—compared to 55.18% for lay persons—and 7.88% of matching prints as ‘non-matching’ (false negative errors).35 In a similar experiment that made use of genuine crime scene prints, where the ground truth is uncertain, they found that examiners incorrectly declared 1.65% of similar non-matching prints as ‘matching’ (false positive errors)—compared to 55.73% for lay persons—and 27.81% of matching prints as ‘non-matching’ (false negative errors).36 In another controlled fingerprint matching experiment, Ulery et al. found that examiners incorrectly declared 0.1% of similar non-matching prints as ‘matching’ (false positive errors) and 7.5% of matching prints as ‘non-matching’ (false negative errors).37 These results demonstrate that qualified, court-practicing fingerprint examiners were far more accurate (and more conservative) than laypersons, and that the rate of false positive errors (i.e. incorrectly reporting that non-matching fingerprints match) in these experimental matching tasks was around 1% and the rate of false negative errors (i.e. incorrectly reporting that matching fingerprints do not match) ranged from 8% to 28%. For criminal justice systems that have routinely relied upon fingerprint evidence for convictions and pleas, these preliminary results should come as a great, if necessarily partial, relief. They suggest that fingerprint examiners have genuine expertise in discriminating between prints that match and those that do not.
In conjunction with the findings and recommendations in the various reports, these studies provide a platform upon which to begin reforming the way opinions about fingerprints are represented and used in legal settings. Our proposed guide to interpreting forensic science testimony begins to address some of the conspicuous deficiencies in contemporary fingerprint practice, especially in the reporting and explanation of results. This proposal attempts to take seriously some of the de-stabilizing epistemic and organizational problems raised in scholarly critiques and the recent authoritative and independent inquiries and reports.
We aim, with this proposal, to enhance legal performance by directing attention toward actual abilities, based on emerging evidence. It is clear that fingerprint identification cannot be regarded as an infallible ‘methodology’ that is detached from human judgement.38 Given the long history of claims about uniqueness, individualization, and a disembodied identification processes, examiners and their institutions should now begin to replace traditional practices and reporting with evidence-based claims that reflect actual capabilities.39 Regardless of what forensic scientists do, criminal courts have a principled obligation to truth and justice.40 Courts, particularly those jurisdictions with a reliability-based admissibility standard, have an obligation to require forensic scientists to present their evidence in ways that embody actual capabilities. This requires evaluating reliability and conveying limitations clearly to the tribunal of fact.
In addition, we take seriously concerns, such as those recently voiced by the Law Commission of England and Wales, about the criminal trial and its limitations with expert opinion evidence.41 The historically accommodating response to fingerprint evidence, the few substantial challenges, and the vanishingly small number of appellate decisions suggest a legal reluctance (or inability) to appreciate the significance of problems with fingerprint evidence.42
Through the provision of a guide, it is our intention to integrate some of the recommendations and emerging research to produce a serviceable tool to assist the legal regulation and use of fingerprint evidence. The Guide is intended to help with the expression and interpretation of opinions about fingerprints by locating them within the appropriate research matrix. We envisage that a version of the Guide would be appended to expert reports prepared by fingerprint examiners, although we also envisage an updated Guide available through a publicly accessible repository.43 The Guide represents a pragmatic attempt to acknowledge and explain actual abilities as well as non-trivial limitations with fingerprint evidence. It is intended to recognize the existence of genuine expertise in comparison work, expose the weak decision-making framework and problem of extrapolation (i.e. the ‘leap’ from match to identification), as well as address the historical reluctance to make appropriate concessions in reports and testimony.
3. Insights from medicine: the diagnostic model
In modern diagnostic medicine, the accuracy of a test is inferred from controlled experiments. In home pregnancy testing, for example, a pregnancy test produces a result that reads ‘pregnant’ or ‘not pregnant’—based on the level of human chorionic gonadotropin (hCG) in urine used as a marker for pregnancy—which may or may not agree with the true state of the world. The validity, reliability, and accuracy of the test come from the aggregation of many controlled experiments. So, in a particular case (i.e. when a woman takes a pregnancy test) we can use this aggregated information to infer something about her true state.
Who compared, Fig. 1A depicts the results from an experiment by Tomlinson et al.44 comparing the accuracy of six home pregnancy tests available over-the-counter. The numbers in Fig. 1A represent groups of women who were pregnant or not and who took the Answer™ home pregnancy test,45 which either resulted in a reading of ‘pregnant’ or ‘not pregnant’. One hundred and twenty pregnant women were given the Answer™ home pregnancy test, 98 of the tests correctly read ‘pregnant’ and 22 incorrectly read ‘not pregnant’. Similarly, 120 women who were not pregnant were given the Answer™ home pregnancy test, 2 of the tests incorrectly read ‘pregnant’ and 118 correctly read ‘not pregnant’.
If a woman purchases an Answer™ home pregnancy test from the chemist, and tests herself, what could she conclude on the basis of the experiment by Tomlinson and colleagues? If the home pregnancy test read ‘pregnant’ in this particular case, whether the woman is in fact pregnant (like the 98 for whom the test produced the correct reading) or whether she is not (like the 2 for whom the test produced the incorrect reading), we do not know. Similarly, if the home pregnancy test read ‘not pregnant’ in this particular case, whether the woman is in fact pregnant (like the 22 for whom the test produced the incorrect reading) or whether she is not (like the 118 for whom the test produced the correct reading), we do not know. This uncertainty does not render the woman helpless; rather, the information could inform her interpretation of the test result (and the question of pregnancy).
We can apply the same diagnostic model to the experiment by Tangen et al. on the matching performance of court practicing fingerprint examiners. The results from this experiment are depicted in Fig. 1B. A group of 37 qualified fingerprint examiners examined 444 pairs of fingerprints from the same person, 409 were correctly declared as a ‘match’, and 35 were incorrectly declared ‘no match’. Similarly, the examiners examined 444 pairs of fingerprints from different people, 3 were incorrectly declared as a ‘match’, and 441 were correctly declared ‘no match’.
If a juror hears an examiner give an opinion about whether two prints match (or not) in a criminal case, what could the juror conclude on the basis of the experiment by Tangen and colleagues? If the examiner declared a ‘match’ we do not know in this particular instance whether the prints are from the same source (like the 409 for which a ‘match’ opinion was correct) or whether the prints are not from the same source (like the 3 for which a ‘match’ opinion was incorrect). Similarly, if the examiner said “no match” we do not know whether the prints are from the same source (like the 35 for which a ‘no match’ opinion was incorrect) or whether the prints are not from the same source (like the 441 for which a ‘no match’ opinion was correct). This uncertainty does not render the juror helpless; rather, the information could inform his interpretation of the examiner's opinion (and the question of source).
We suggest that an indication of performance (and error) in previous situations, (reasonably) similar to the particular analysis, provides potentially valuable information to those obliged to evaluate fingerprint testimony. This ‘statistical base rate’ is general information. The juror can reason with this information to infer something about the particular case—to deduce the particular from the general. The juror can also use information about the particular case (‘causal base rates’) to temper these judgements, if they think the information is relevant. Judgements can be anchored to a plausible base rate and tuned by reasoning, informally, about the information specific to the particular case.46
Broadly, the diagnostic model is an approach that offers information about similar situations in order to help decision-makers reason about the present case. The goal for a diagnostic model applied to forensic testimony is to give information to the legal participants to assist their decision-making around admissibility, challenges to evidence, instructions and warnings, and for the jury around the value of evidence and the ultimate conclusion. The goal is to provide information in a way that will help the jury to weigh the evidence, evaluate the arguments, and to judge the degree of belief warranted by the information presented.47 Often this will involve information about general, or indicative, error rates and practical limitations. As in the diagnostic model, much of the information (i.e. scientific evidence) that can be presented will be based on general data from previous studies (i.e. from beyond the instant case) and the legal participants and fact-finder must reason and make inferences from the general to the particular case.48
Little is currently known about the types and forms of information that will assist triers of fact to make optimal decisions. The Guide is presented as a pragmatic intervention: an evolving compromise that endeavours to provide a diagnostic-style framework to improve forensic reasoning. An in-depth treatment of the diagnostic model applied to forensic testimony is forthcoming, but our goal, in the first instance, is to help to ensure that expert evidence is presented in ways that are scientifically tenable. This involves embodying its known value and disclosing limitations in ways that help triers of fact to make sensible decisions about the forensic science evidence in a particular case.
4. A guide to forensic testimony: fingerprints
This section provides an example of what a guide for fingerprint evidence proffered for identification might look like at this stage. That is, the kind of information or caveats that ought to be included with the fingerprint examiner’s report and testimony. It is an intentionally short document that places emphasis on brevity, comprehensibility and the goal of capturing both the considerable evidentiary potential as well as known limitations. Requiring ongoing revision—at least until there are sufficient studies to support a stable consensus—this preliminary version is based on the few scientific studies that have assessed the performance of fingerprint examiners in circumstances where conditions were deliberately controlled. In the remainder of this article we endeavour to unpack the Guide and some of the implications of the recent reports and emerging studies in ways that are sensitive to the criminal justice milieu, and especially the criminal trial.
A decision about whether two fingerprints match or not is based on the judgment of a human examiner, not a computer.
There are several documented cases where an examiner has incorrectly said that two prints ‘match’ when they actually came from two different people. Laboratory-based experiments suggest that errors of this sort happen infrequently (around 1% of the time).
In practice, however, it is unknown how often examiners say that two fingerprints match when they actually come from two different people.
Without specific evidence, it cannot be known whether an error has occurred in a particular case.
For further information see www.InterpretingForensicTestimony.com
5. Specific issues arising from the Guide
This proposal is intended to begin the process of practically reforming the way fingerprint (and other types of comparison) evidence is presented—in reports and testimony—and evaluated in courts. While the Guide may be controversial—with fingerprint examiners, other forensic scientists and commentators—it will be virtually impossible to generate consensus in this area. Even in the absence of complete consensus, the fundamental nature of problems with the comparison sciences identified by the NRC, the EWGHF and Lord Campbell, in conjunction with the espoused goals of criminal justice (particularly rectitude, the concern with protecting the innocent, and the need for fair criminal proceedings), mean that we should not persist with our current practices. There is a need to develop some empirically based mechanism to constrain the way opinions about fingerprints are expressed and improve the way lawyers, judges and jurors evaluate fingerprint evidence. Current legal practice tends to be either indifferent to criticism, or has attempted to craft responses on the run. These responses are generally defensive (of past legal practice) and not genuinely engaged with the reports, their recommendations and (emerging) empirical evidence—see Section 6.
There are many difficulties associated with the attempt to ascertain the value of fingerprint evidence, both generally and in specific cases. The commitments of our criminal justice systems, in conjunction with authoritative disclosures about fingerprint (and other forensic science) evidence, however, would seem to dictate the need for the state to present its incriminating expert evidence in ways that embody the value of the evidence. Unavoidably, this requires the proactive disclosure of limitations. Forensic science evidence cannot be admitted on the basis that scientifically notorious limitations are a matter for the trial and weight—to be drawn out through cross-examination, rebuttal experts and judicial instructions—should the evidence be contested. Rather, when it comes to comparison techniques in routine use, it would seem incumbent upon the state to support proffers with evidence of ability and accuracy, and to clearly and effectively consider and explain limitations.
In many areas of forensic science and medicine, levels of error are knowable yet unknown even though there are sometimes good reasons to think that practices and interpretations are error prone. Facial and body mapping, gait analysis, voice comparison, bite marks, blood spatter, document comparison, as well as foot and shoe prints, tool marks, ballistics/firearms, soil, and non-DNA hair comparison are all conspicuous examples of forensic techniques that appear to lack the requisite research (or empirical) foundation.49 In these areas, the accused is typically left to expose limitations with techniques, as well as the performance of investigators, retrospectively rather than require the state to have studied techniques and tested examiners in order to provide some indication of the existence of expertise and the kinds of errors that are associated with analyses and inferences. While the Guide targets fingerprint evidence, we can imagine similar and in some ways generic versions applied to virtually all expert reports and testimony concerned with identification or sourcing.50
As a heuristic, a guide has the added benefit of informing the use of forensic science and medical evidence not only at trial and on appeal, but also during investigations, decisions to prosecute, and in plea negotiations. In this way, the Guide is practically oriented to the needs of the criminal justice system and its various personnel. In relation to criminal prosecutions, the Guide places both the prosecutor and the accused in an equivalent position vis-à-vis the reliability of a technique and the probative value of the evidence. It is, in addition, intended to discipline forensic scientists (in court) such that they restrict their opinions to assertions that can be supported by current scientific research.
5.1 Experimental evidence and the limitations of empirical error estimates
So far, experiments on the matching accuracy of examiners have been tightly controlled and deliberately artificial in order to balance fidelity, generalizability, and control.51 They were not designed to resemble the everyday operations of a fingerprint bureau. For example, the examiners studied by Tangen and his colleagues were prevented from making ‘inconclusive’ judgements, they did not have their usual tools available to zoom, rotate, or apply filters to images, there was no peer assessment or ‘verification’ of the prints, and so on. On the other hand, the examiners were not provided with any contextual information about the demographics of the source individual, the severity or nature of the case, or attributes or conditions of the latent print, which could potentially sway their judgement. Moreover, the prints were from known sources—i.e. there was no uncertainty about ground truth.52 Consequently, the error rates incorporated into this first iteration of the Guide are based entirely on the performance of examiners within controlled environments, and limited to assessing the ability to discriminate between matching and non-matching prints.
Errors, however, can arise at any stage of the process, from collecting latent prints at the crime scene to providing testimony in court. The frequency, severity, and kind of error may depend on factors such as an examiner’s training and experience, the nature of peer review, the quality of the recovered print (or image), the type of surface and method of retrieving or imaging, the efficiency of the search algorithms used to retrieve the corresponding ten-print candidate, as well as a range of formal and informal practices such as exposing analysts to prejudicial, though domain irrelevant, information.53 Several experiments are currently being conducted on various aspects of an examiner’s workflow across different laboratories, and will ultimately measure the performance of entire pre-trial systems. These will be invaluable for refining estimates of error and should be incorporated into subsequent iterations of the Guide.
It might be argued that the available studies are too limited, lacking ecological validity, to begin to reform the presentation of fingerprint evidence. They do not, after all, address the leap of faith inference from matching to identification.54 Our response is as follows. First, we are drawing upon the recommendations of authoritative reports by well-credentialed independent bodies. Secondly, although preliminary and likely to be refined, the validity studies by Ulery et al. and Tangen et al. (and Wertheim et al., Langenberg et al., and Dror et al.), are consistent and all we have in the way of scientific studies at this point in time. Thirdly, failing to respond to the recommendations and insights about the performance of fingerprint examiners maintains the status quo and the dangers associated with equating opinions about matches with error-free identification. Finally, only carefully controlled experiments have the precision and capacity to isolate the conditions and causes of mistakes, and contribute to the development of a system that is resilient to error; making it harder for people to do something wrong and easier for them to do it right—see NRC 3, LPEHF 9.1, 9.2.55
5.2 Uniqueness, variability, and error
Uniqueness and persistence are necessary conditions for friction ridge identification [i.e., fingerprint comparison] to be feasible, but those conditions do not imply that anyone can reliably discern whether or not two friction ridge impressions were made by the same person. Uniqueness does not guarantee that prints from two different people are always sufficiently different that they cannot be confused, or that two impressions made by the same finger will also be sufficiently similar to be discerned as coming from the same source. The impression left by a given finger will differ every time, because of inevitable variations in pressure, which change the degree of contact between each part of the ridge structure and the impression medium. None of these variabilities—of features across a population of fingers or of repeated impressions left by the same finger—has been characterized, quantified, or compared.56
Examiners have previously claimed that fingerprint identification is infallible and that there is a zero error rate for fingerprint comparisons.57 These assertions are typically justified by reference to the uniqueness of prints and their longstanding use for purposes of identification.58 Nevertheless, examiners make mistakes. Courts should recognize that errors are not due to people having identical (i.e. non-unique) fingerprints; errors are due to examiners incorrectly matching prints that are not from the same source and failing to match prints that are from the same source.59
In many cases, latent prints can be used to assist with identification by means of their classification as a ‘match’ or ‘non-match’. Qualifying the meaning of a match with an indication of error is intended to draw attention to the non-trivial risk of a mistake in the process of classifying two prints as a match or non-match. In the absence of more accurate comparison practices, or information about the value of apparent matches, such an approach endeavours to reinforce the evidentiary value and reliability of latent fingerprint evidence while recognizing real limitations—including uncertainty around the inference to identification (or ‘leap’). The expression of an indicative, or general, error rate recognizes that comparison processes are fallible in circumstances where we are not entirely sure what a match actually means.
Opinions about the uniqueness of fingerprint features must be considered against the demonstrated abilities of examiners. For example, there is no evidence that examiners can judge the statistical base rate of particular fingerprint configurations. Even if they could, there is still a degree of human error associated with such judgements. Such abilities and the level of performance should be empirically demonstrated rather than asserted. Common problems with claims of uniqueness persist, in that examiners have to agree with each other (and themselves) on what counts as a feature, acceptable variations of these features must be specified, and there needs to be a database to draw from that is impervious to the noise and ambiguity that is intrinsic to crime scene prints—see LPEHF Rec. 3.6. In the end, any judgement about the relative occurrence of particular features is a human judgement that is unavoidably prone to error. Claims about a ‘methodology’ that is detached from human judgement must necessarily be ‘off limits’.
5.3 Towards identification: what might a ‘match’ mean?
There are two separate issues pertaining to the presentation of fingerprint evidence that have been conflated historically. Through their concentrated focus, the validation studies help to distinguish them. The first involves the ability of examiners to match a pair (or set) of prints. As we have explained, trained and experienced examiners tend to be good at this, but they are not free from interpretative error. Second, is the issue of what a declared match means in terms of identification. We have referred to this as the inferential leap of faith. The second issue is quite complicated, and there is little evidence that examiners have relevant expertise. Historically, fingerprint examiners assumed that all humans have unique fingerprints (and implicitly, that all the prints they were willing to characterize as ‘matching’ were produced by a single identifiable source) and, in consequence, if they declared a match they had identified a particular individual to the exclusion of all other persons. As the reports explain, these assumptions are neither empirically based nor plausible. While more work needs to be done on fingerprint comparisons and the conditions in which they are made, the main problem at this stage is moving from a match decision to the attribution of significance in terms of identification. Patently, the declaration of a match does not equate with positive identification. However, on average, a declared match will be probative on the question of identity. The dilemma is how can we make sense of the match evidence. That is, how should we evaluate a putative match?
Courts have experimented with several methods of managing this problem—not only in relation to fingerprint evidence. Apart from probabilistic approaches, associated with DNA evidence, two of the more prominent methods are to limit testimony to the mere description of similarities (so-called ‘splitting’) and the use of verbal scales.60 The first approach is conspicuous across a range of emerging comparison sciences. Judges, concerned about underlying evidence, particularly the distribution and frequency of features (whether sub-features of fingerprints or facial features in image comparison), have on occasion-restricted analysts to describing similarities and/or differences. Such an approach would enable a fingerprint examiner to report a match, but to say no more. They would be unable to venture an opinion on the significance of similarities between two prints.
For a variety of reasons this is not an appropriate response to latent fingerprint evidence. First, in the absence of information about the distribution and relationship between fingerprint minutiae, and particularly a range of issues related to differences between all (including same source) fingerprints and fragments of fingerprints, the move from similarities to positive identification is problematic.61 Secondly, the cultural familiarity with fingerprint evidence, and equating a match with positive identification, is such that describing similarities—regardless of the precise nomenclature—will be a de facto identification and understood as positive identification.62 On policy grounds, thirdly, fingerprint techniques have been around for so long that it seems inappropriate to simply excuse the tardy performance of examiners in this way. Allowing fingerprint examiners to express opinions about similarities does not address the issue of their rate of error in matching, nor moderate the significance of any match relative to identification. It also presents the jury with a series of similarities without providing rational means of attaching significance.63 In theory, splitting, removes the probative value of the evidence (to a point that threatens its logical relevance), and in practice it continues to imply positive identification.64
A second approach, enabling forensic scientists to go beyond merely describing similarities, is the use of a verbal scale to attach evidentiary significance to the match (or similarities). Image comparison evidence, so-called facial mapping, is a good example. In the absence of DNA-style databases or information about the distribution and independence of facial features, image comparison witnesses have been allowed to move from alleged similarities (or matches) between persons in images to opine about their significance in terms of identification. In Australia, though formally restricted to the description of similarities (and, in theory, differences), there has been slippage with image comparison witnesses testifying in terms of ‘high level of anatomical similarity’.65 In England and Wales, where there are no such prohibitions on positive identification, in recent years image (and other types of) comparison witnesses have adopted verbal scales, such as the one reproduced in Fig. 2, to express their incriminating opinions.66
Apart from criticizing the use of numbers to rank the verbal equivalents (in Atkins), English courts have tended to accept such expressions provided the jury is informed that they are not derived from a database.67 The problem, of course, is that the move (or leap) from alleged similarities in facial features to an indication of the value of those similarities—such as ‘lends strong support’—as evidence of identification does not have empirical grounding. Those comparing faces and fingerprints are not (at this stage) conversant with the frequency and independence of features and sub-features. As with latent fingerprint examiners, we do not know if they possess expertise in assigning a particular level of significance to apparent similarities.68 In consequence, it is not clear what probative value(s) we should attach. While some formulations, such as those incorporated in the table (above), might, as qualifications, constitute an improvement over positive identification, this ‘solution’ remains impressionistic, speculative and quite likely misleading. We contend that the use of verbal scales is inappropriate because it relies on the examiner’s impression and privileges untested ‘experience’. It cannot be readily assessed and it is not easy to explain the methodological frailties at trial—especially when it is the accused challenging the opinion of an experienced analyst.
One possible compromise is to restrict testimony to similarities (or even ‘matches’) and incorporate information about the error rate in comparisons while endeavouring to explain that we do not have an established means of moving between the declaration of a match and the attribution of evidentiary significance. This would involve reporting a ‘match’, an error rate with matching (and possibly other parts of the process when empirical evidence emerges), and informing the jury that a match is not the same as identification. That is, we currently do not know what the relationship is between a match decision and positive identification. The tribunal of fact should not be allowed to attach any significance they want in the absence of information about the indicative value of the opinion. This is the basic approach proposed in the first iteration of the Guide. The important point is to introduce the error rate, rather than rely upon restricting opinions to similarities given the pervasive belief that fingerprints are unique and that a declared (or reported) match is the equivalent of positive identification.
5.4 Probabilistic evidence, likelihood ratios and error
There has been an enduring push to ‘objectify’ fingerprint comparisons in a similar way to the analysis and presentation of DNA evidence.69 The methodology is fairly straightforward. An examiner can locate and connect a set of features in a fingerprint, which form a polygon where the length of each side and the angles within the polygon are defined numerically. The same measurements are calculated for a second fingerprint. Two hypotheses are then compared: (1) that the two prints come from the same source (i.e. the differences between the polygons are due to the partial or degraded nature of the prints); and (2) that the two prints come from different sources (i.e. the similarities and differences between the polygons are coincidental). One score is then calculated for the first hypothesis by computing the extent to which the same polygon can vary based on a range of marks from the same source. A second score is calculated for the second hypothesis by computing the rarity of this particular polygon compared to polygons that are derived from other sources that have been selected at random. The larger the ratio of within- to between-polygon variation, the stronger the evidence for the first hypothesis compared to the second.
There are several problems with this specific methodology as well as the general approach to providing rarity values in testimony. First, the basic units of analysis in these computations are the distances, directions, and angles among particular configurations of basic fingerprint features (e.g. ridge endings and bifurcations). With highly degraded prints—the sort commonly lifted from crime scenes—there will be very limited consistency among examiners in what counts as a landmark, where they are located and how many to include. These will vary greatly depending on a variety of contextual and human factors including expertise, traditions, fatigue, time constraints and so forth. As a result of this variability, the configurations of the polygons may vary markedly between examiners and even for the same examiner in different circumstances. Indeed, the Automated Fingerprint Identification System (AFIS) currently outputs a numerical measure of similarity based on the minutiae, features, directions, and spatial relationships of the fingerprints that the operator submits to the system.70 The magnitude of these measures varies greatly depending on the number and nature of the landmarks that each examiner provides, and one can easily cherry pick particular parameters to produce a large or small value. The same problem with consistency is almost certainly true for any probabilistic model that involves human judgement, which tends to subvert the purpose of an objective system. Secondly, the score that is derived to test whether the similarities and differences between the polygons are coincidental is based on polygons that have been generated from other sources selected at random. The measure of ‘coincidence’ may therefore be misleading, particularly when an examiner is faced with the most highly similar candidate print that is retrieved from the database of tens of millions of possibilities. An appropriate measure of coincidence here ought to reflect the confusability of highly similar candidates, which is common in practice. The rarity scores generated by these models may well assist examiners in comparing fingerprints by reducing the amount of time it takes to arrive at a decision, or improve the overall accuracy of their judgement (which is testable), but they are of little use on their own. Thirdly, as discussed in Section 2, the formal training of fingerprint examiners tends to be specific to the workplace conditions (e.g. principles and foundations of friction ridge examination, biology and physiology of friction ridge skin, history of fingerprints).71 Examiners do not have a background or demonstrated expertise in probability theory, statistics, or mathematics enabling them to testify about the assumptions and calculations behind these probabilistic models.
Even if an examiner was perfectly consistent in his own analyses of fingerprint landmarks on different occasions (and other examiners agreed), if the resulting probabilities were sufficiently conservative and adequately measured the rarity of the prints, and if the examiner fully comprehended the mathematical basis of the model and effectively communicated it to the court, the problem with the general approach to providing rarity values in testimony remains—in that these values sidestep the very real possibility of human error. Recognition of a real error rate provides an important means of qualifying an expert’s interpretation.
The inclusion of an indication of error would seem to place expert opinion evidence in an empirically based format that is conducive to the trial framework and comprehension by lawyers, trial judges and juries.72 Error rates enable opinion evidence to be presented and interpreted in a manner consistent with fundamental concerns about reasonable doubts. The provision of an indication of error helps to ground expert opinions; helping to prevent prosecutors and judges from simply assuming that the match—particularly claims about uniqueness, infallibility or improbably small random match probabilities—is infallible or that interpretation is mechanical, that protocols were followed, that labelling was accurate, that equipment worked, that interpretations were sound, and reviews effective.73 While, the reported error rate will not necessarily capture error in the particular case, it provides a useful (and arguably necessary) background rate against which claims about matching can be assessed.74
Recognizing and incorporating errors into the reporting of results may, with the assistance of further study, enable fingerprint examiners to move beyond ‘match’, ‘non-match’ and ‘inconclusive’—to the extent that they are willing to concede greater uncertainty and probably greater risks of error—see LPEHF Rec. 3.8. What the criminal justice system, particularly courts, should do in relation to less reliable forms of incriminating opinion warrants detailed consideration against admissibility standards and discretions such as those oriented to the danger of unfair prejudice to the accused.75
5.5 Improving performance and reducing bias
It is vital that examiners are not begrudging or disingenuous in their recognition and attribution of possible errors—both generally and specifically. The failure to accept or recognize the existence of errors distorts their evidence, and makes it more difficult to improve processes and performance. It also shifts responsibility to the accused to somehow identify errors made in the process of recovery, collection, analysis, interpretation and reporting of opinions retrospectively during an adversarial trial.
Given the interpretive (or subjective) nature of comparisons, the various reports recommend undertaking research into bias and developing practices that prevent the detrimental effects it may have on analysis—NRC Rec. 5, LPEHF 3.3, SFI 7, 8 and 9. Fingerprint examiners are, in most bureaus, not insulated from case information when undertaking their analyses or even when selecting prints for comparison from fingerprint databases. This means that their comparisons may have been influenced by information that is not relevant to their analysis, such as the criminal record of suspects with prints similar to those recovered or the fact that the accused made a retracted confession or was believed by investigating police to be guilty.76 Generally, given the absence of empirical evidence, it seems appropriate to withhold such information from analysts, even if only until they have made and recorded their first analysis of the evidence—NRC Rec. 5, SFI 8.77 Exposure to domain irrelevant information should be documented, and courts should be interested in such exposures when considering both admissibility and weight. It may be that we should consider excluding comparison evidence where the examiner has been unnecessarily exposed to gratuitous information.78 Matches (and non-matches) declared after exposure to domain irrelevant information are at risk of having been influenced by that information. Where the examiner has been exposed to prejudicial information, any match decision cannot be understood as independent corroboration and should not be presented as such at trial.
5.6 Individual performance
The error rates reported in the Guide are based on controlled experiments of the matching performance of qualified fingerprint examiners. Given the dearth of research on the competency of examiners at all stages of fingerprint analysis, or on the factors that influence performance, and in the absence of comprehensive (i.e. industry wide) measures of accuracy, the performance of examiners in these experiments can only be used as a generic or indicative rate for the field. The error rate does not apply to individual examiners or even particular fingerprint bureaus. In terms of individual performances, we can assume that they vary—between examiners, as well as over time and conditions—but we do not know which examiners, bureaus, work contexts, or situations are more or less error prone. This is part of the problem. Most forms of practice do not involve making comparisons in controlled conditions. In principle, it seems useful to have an indication of a general error rate for comparisons (and other parts of the process).79 These can be used as a base rate, even if there are arguments about the performance of the individual examiner and the value of the prints in specific cases.80
Here, it is important to distinguish the proficiency tests currently used by fingerprint examiners, such as those provided by Collaborative Testing Services, Incorporated. These tests have been trenchantly criticized and are insufficient for measuring accuracy.81 In order to make general claims about accuracy—beyond those of specific prints at a specific level—different (and randomized) sets of prints for each examiner are needed. Commercial proficiency tests do not adequately address the general issue of expert matching accuracy and were not designed to disentangle the factors that affect matching accuracy.
6. Trial safeguards are not a viable mechanism for managing speculative opinions
Regardless of the admissibility standard, whether focused on ‘specialized knowledge’ (and reliability), a field of knowledge or assisting the tribunal of fact, the opinions of fingerprint examiners, about whether two prints match or do not match, should ordinarily be admissible because matches appear to be accurate and probative on the question of identity.82 Admissibility should be subject, however, to the disclosure of limitations (i.e. an error rate and exposure to potentially biasing information and breaches of standards and protocols) and the witnesses restricting their report or testimony to areas in which they possess demonstrable expertise. The state should be obliged to study techniques in widespread use and to proactively concede limitations.83 Drawing on the various recommendations, we would contend that latent fingerprint evidence should not be admitted at trial unless known limitations and measures of performance are included with the opinion.
The Guide is also motivated by emerging evidence that trial safeguards are not consistently effective in exposing and conveying limitations with incriminating expert opinion evidence.84 Appellate review has also been ineffective at exposing errors or even overconfidence.85 Common law courts have been remarkably accommodating toward the opinions of state-employed fingerprint examiners and other forensic scientists.86 This accommodating posture may have produced complacency and overconfidence (among lawyers, forensic scientists, judges, jurors and the public), and may have contributed to the remarkable paucity of research in many areas of forensic science. While limitations might be explored on the voir dire, or via cross-examination, rebuttal witnesses and judicial directions during the trial, such safeguards have not proved to be effective in exposing limitations or identifying real risks of error. Significantly, problems with fingerprint evidence did not emerge from the crucible of the trial.
Trial counsel and judges should be more attentive to expert evidence (and its bases).87 The Guide is intended to help in this regard. We should not discount the fact that fingerprint examiners continue to characterize a ‘match’ as positive identification or something practically indistinguishable. To the extent that qualifications have occasionally and sporadically arisen they are often begrudging and dismissive—e.g. reluctantly conceded as hypothetical (or theoretical) possibilities (LPEHF 6.3)—or transformed into (un)ambiguous similarity evidence.88 While many examiners are working on improving practices and should be cooperating with scientists to change how comparisons are undertaken and interpreted, experience suggests that we cannot rely on the fingerprint community to unilaterally reform their practices. The Guide is intended to help stimulate this transformation while providing a framework that enables legal institutions to utilize fingerprint evidence on a more empirically secure footing.
7. ‘Expertise’: a practical ability, tacit or explicit knowledge?
An interesting aspect of the emerging research on the performance of fingerprint examiners is that their judgements and decisions may not be (fully) explicable.89 Many jurisdictions, such as those based on the Federal Rules of Evidence (FRE) in the US and the uniform evidence law (UEL) in Australia, specify the need for knowledge—‘scientific, technical and other specialized knowledge’ and ‘specialized knowledge’, respectively—in their admissibility standards for expert opinion evidence. It may be that the abilities or expertise of examiners is not readily reducible to articulable knowledge or a particular method, but attributable to learned abilities to compare prints and recognize apparent similarities and differences. Whether this is actually ‘knowledge’ is an interesting question, though there is little doubt that comparisons by trained and experienced examiners are generally reliable.90
Research on medical decision-making has shown that in some circumstances the factors (purportedly) relied upon by experts may not be clearly defined or verbalized, yet performance may still be accurate.91 That is, the information that examiners think they rely on, or use to rationalize their decision-making (retrospectively), may bear limited resemblance to the information that they actually use. Processing may not, in fact, be entirely conscious. In contradistinction to legal practice and assumptions about expertise (based on ‘knowledge’), if comparisons are based, to some non-trivial extent on intuitive or unconscious processes, then it may prove to be very difficult to expose error at trial and the analyst might remain sincerely confident even when mistaken.92 Conventional trial safeguards, such as cross-examination may, in consequence, be of limited value in exposing limitations or errors in such circumstances—see Section 6.
8. Jury access to images and working notes
When asked to compare and match fingerprints laypersons are quite error prone. Vokey et al. demonstrated that novices generally have the ability to match prints, but their performance deteriorates dramatically when it comes to distinguishing highly similar but non-matching prints; such as those obtained through database searches.93 Tangen et al. found that laypersons incorrectly declared 55.18% of these similar non-matching prints as ‘matching’, compared to 0.68% for examiners.94 Lay vulnerability to error raises enduring questions about whether the tribunal of fact—jury or trial judge—should be allowed to examine and compare prints.95 Empirical studies might be interpreted such that they counsel against allowing laypersons to undertake their own comparisons.96 Jurors and judges will be particularly vulnerable to error, and this will be accentuated by: the lack of feedback; exaggerated confidence in their own abilities; and exposure to (potentially biasing) contextual information. This ought to lead judges to exclude images of prints, working notes and mark-ups unless the interests of justice demand admission or the defence seeks their introduction. Lay impressions, about whether two prints match or do not match, are likely to be error prone and should not generally be encouraged. Given the levels of error, there are real dangers in allowing the jury to undertake its own interpretation of contested prints and admission of such images raises the real, and perhaps insurmountable, risk that this will occur.
9. Reports rather than testimony?
Because of the many, very real complications with fingerprint evidence and the danger of opinions being presented in terms that exceed what can be empirically sustained, a slightly more radical response might be to limit the presentation of fingerprint evidence to a short documentary form embodying the kinds of issues identified in the Guide.97 That is, in most cases the examiner should not testify in person and the prints and working notes should not be adduced or admitted. These documents should, of course, be disclosed to the defence. Such an approach would not only help to prevent testimonial misrepresentations, but it would also save time and resources. Again, where the defence requires attendance, or where attendance is in the interests of justice, the examiner should testify in person.98
The proposed Guide represents a pragmatic attempt to address criticisms of fingerprint methodology and the way most comparisons are currently reported and explained. It is intended to begin to address and recognize the existence of genuine expertise in undertaking comparison work, the weak underlying decision-making framework, and the historical reluctance among examiners to make appropriate concessions in reports and testimony. If this approach is conceived as excessively empirical (or requires too much evidentiary support), we would remind the reader that for a century courts have allowed techniques to be misrepresented even though they could have been studied and improved. Along with the National Research Council of the United States National Academy of Sciences, the National Institute of Justice and the National Institute of Standards and Technology (US), Lord Campbell (Scottish Fingerprint Inquiry), and others, we are supporting the introduction of accountability mechanisms in forensic science reporting and testimony.
At the base of this proposal is a growing chorus of criticism about the kinds of studies and evidence that should underpin the forensic sciences—at least, when they are relied upon in criminal proceedings. We, along with others, believe that courts have an obligation to require evidence of ability—actual expertise and rates of accuracy—particularly about forensic science and medicine techniques in routine use. Empirical studies provide evidence of ability, they enable those evaluating the evidence to have a clearer idea of the value of evidence than is usually provided through cross-examination or judicial cautions, and the studies will often inform the manner in which experts should be allowed to express their opinions as well as the scope of their testimony. All of these can be observed in the recent experiments on the abilities and accuracy of fingerprint examiners.
The Guide, and particularly the focus on error rates, offers a useful means of assessing and regulating a range of comparison practices, especially those where the likelihood of generating useful probability rates seems remote or even unlikely. Even if fingerprint examiners and others develop probabilistic approaches, there will be a need to consider how error rates should be incorporated into the results, as the various reports recommend. For many other types of comparison practices (e.g. images, voices, ballistics, tool marks, bites, footprints, tire prints, pattern marks and so on), error rates will provide important insights into the value of the evidence—by highlighting the abilities of the witnesses—that enable judges (and lawyers) to determine the admissibility of opinions and their weight should they be sufficiently reliable for admission. Preliminary studies suggest that not all comparison and identification techniques will be as accurate as fingerprint.
We accept that, notwithstanding its empirical sensitivities, this is a pragmatic response or compromise. We also accept that others may have alternative, perhaps better, ideas about how we can respond to the frailties of latent fingerprint evidence. Our proposal is based on what we currently know, empirically, about latent fingerprint evidence in combination with the realization that investigators and courts are unlikely, and perhaps unable, to respond unilaterally to latent fingerprint evidence. No doubt many examiners, and others, will be concerned about even these modest, empirically inflected impositions. While acknowledging that the Guide and the imposition of an indicative error rate represents a considerable departure from historical assumptions and practices, there is no reason to continue to admit and accept opinions about latent fingerprints in the conventional accommodating manner. Our proposal accepts that latent fingerprint evidence is potentially powerful evidence of identity, and we believe that the Guide provides a compromise that reflects current knowledge and abilities, as well as what we now know about the limitations of fingerprint evidence (as well as the limitations of trials and appeals).
The legal system should not avert its eyes to mainstream scientific consensus around frailties and errors in the way comparison sciences are practiced and reported. Disregarding scientific consensus threatens the legitimacy of legal institutions and undermines their ability to deliver accurate verdicts and, simultaneously, justice.99
This work was supported by an Australian Research Council (ARC) Future Fellowship (FT0992041) to Edmond, an ARC Linkage Grant (LP120100063) to Tangen and Edmond and a Fulbright Scholarship to Thompson (see www.ForensicReasoning.com).