The concept "differential predictability" refers to the idea that people may vary in the extent to which their behavior is predictable by some predictor measure. A previous study showed that freshman engineering grades of students classified as "noncompulsive" were more predictable by Strong Vocational Interest Blank scores than were the grades of "compulsive" students. Two rough indicators of compulsiveness were used: (1) tendency to resemble accountants on the Accountant scale of the Vocational Interest Blank and (2) being above the regression line of reading speed score on vocabulary score for the Cooperative English Test C2: Reading Comprehension. The study was replicated, using freshman students in the School of Engineering at Princeton. The finding that noncompulsive students are more predictable than compulsive students, as judged by correlations between interest blank scores and freshman average grade, seems to hold only for the occupational keys most logically related to engineering (Mathematician, Physicist, Engineer, and Chemist) when the groups are defined on the basis of reading speed relative to vocabulary.