From the authors’ Table 3, one sees that when four or more items on the seven-symptom scale are endorsed, sensitivity equals 0.803 and specificity equals 0.973. In the population evaluated, the proportion of individuals with PTSD, or disorder prevalence, was 142/1,830=0.078. Using this prevalence, Dr. Breslau and colleagues correctly calculated the positive predictive value as (sensitivity × prevalence) / (sensitivity × prevalence + [1 – prevalence] × [1 – specificity]) = 0.713 and the negative predictive value as (specificity × [1 – prevalence]) / (specificity × [1 – prevalence] + prevalence × [1 – sensitivity]) = 0.983. The authors should have pointed out, however, that these values are correct only for the prevalence in their group. The prevalence of PTSD has been reported to range from 1% in community-based samples to 75% among rape victims (3). If the cutoff of four or more symptoms is used in a population where the prevalence is 2%, the positive predictive value is 0.378 and the negative predictive value is 0.996; if the same cutoff is used in a population in which the prevalence is 50%, the positive predictive value is 0.967 and the negative predictive value is 0.832.