The construct validity of the standardized-patient (SP) examination used at Southern Illinois University (Springfield) School of Medicine was assessed by comparing 66 second-year and 70 fourth-year medical students on 5 SP cases. The results show sizable differences between the groups. The usefulness of passing rates the effect-size measures as a means of enhancing the typically weak evidence for validity provided by group-differences studies of construct validity is demonstrated. Results obtained through these approaches show that, as would be expected, the clerkships are having a considerable effect on clinical competence and that the examination is sensitive, as a valid measure of clinical competence should be, to these changes in the clinical-competence construct. A table summarizes passing rates and group means. (Author/SLD)