The diagnostic utility of a test is relative to the base rate of the diagnosis in the population of interest. The extensive use of tests in neuropsychological assessment makes knowledge of the relevance of base rates in this context an important issue. Professional and associate members of the National Academy of Neuropsychology (NAN; n=279) answered questions involving (1) a simple use of base rates in the absence of additional diagnostic information, (2) sensitivity and specificity, and (3) positive predictive value (PPV) presented in either a probability or frequency format. Although the majority of participants answered correctly the simple base rate, sensitivity, and specificity questions, only 8.6% answered correctly the PPV question presented in a probability format versus 63.0% correct using a frequency format. These results are discussed in terms of education about the importance and application of base rate information and difficulties applying base rate information in practice.