In Response: Biological arguments for selecting effect sizes in ecotoxicological testing—A governmental perspective

Extract

Criticisms of the uses of the no‐observed‐effect concentration (NOEC) and the lowest‐observed‐effect concentration (LOEC) and more generally the entire null hypothesis statistical testing scheme are hardly new or unique to the field of ecotoxicology [1, 2, 3, 4]. Among the criticisms of NOECs and LOECs is that statistically similar LOECs (in terms of p value) can represent drastically different levels of effect. For instance, my colleagues and I found that a battery of chronic toxicity tests with different species and endpoints yielded LOECs with minimum detectable differences ranging from 3% to 48% reductions from controls [5].

For interpretations of field studies, recommendations for improved practices include using confidence intervals rather than hypothesis testing for group comparisons and evaluating whether apparent effects exceed predetermined “critical” effect sizes [2, 6]. For interpretations of toxicity tests, recommendations for improved practices emphasize replacing NOEC and LOEC comparisons with either model‐based true no‐effect concentration estimates or curve fitting and from the fitted curve functions, reporting concentrations that produced x% of effects (ECx) [7, 8, 9]. These developments beg the question: What levels of toxic effect are of concern or can be considered negligible? Biologically based arguments for selecting x are scarce, and instead discussions for selecting x from curve‐fitting approaches have emphasized statistical or test performance considerations for selecting x rather than biological implications. Confidence limits, variability of point estimates, model dependence, and comparisons of NOECs to ECx percentages have been evaluated [10, 11]. These statistical considerations are of value but are not sufficient by themselves and may be circular. If a major shortcoming of NOECs is that they may actually correspond with fairly high adverse effects [9, 12], why should it make sense to then turn about and ask what levels of effects are typically associated with NOECs to define the x in ECx values?