In simulation studies, the F test for differences in regression slopes has tended to distort nominal Type I and II error rates when the 2 subgroup error variances exceeded a 1.50:1 ratio. This study examines the frequency and extent that this ratio is violated within data sets relevant to applied psychology. The General Aptitude Test Battery (GATB) validity study database contained ability data and overall job performance ratings. The Project A military database contained both ability and personality data, along with job performance factor scores and an overall job performance rating. Results suggest that subgroup (White-Black, male-female) error variances are often homogeneous enough to support F test results from past empirical work. Enough heterogeneity was found, however, to urge applied psychologists investigating differential prediction to explore their data and consider the possibility of alternative statistical tests.