The test performance of students who took the Scholastic Aptitude Test (SAT) only once as juniors was contrasted with students who took the test as juniors and again as seniors. Estimates of expected test performance on a common initial administration in the junior year were derived from separate equating sections and background variables. Residuals of observed minus expected test scores revealed statistically significant differences between students who took a single administration of the SAT as juniors and those who took the same initial administration but also repeated the test as seniors. The initial observed scores of students later repeating the test were consistently lower than their expected scores for both the verbal and mathematical sections. The results indicate that self-selection occurs when students decide to repeat a test. Score changes among these students reflect negative errors of measurement on the initial test administration. (Author/DWH)