The most commonly used measures of locus of control are Rotter's Internality-Externality Scale (I-E) and Nowicki and Strickland's Internality-Externality Scale (NSIE). A reliability generalization study is conducted to explore variability in I-E and NSIE score reliability. Studies are coded for aspects of the scales used (number of response points, number of items) and for sample demographic descriptors (percentage female, average age). Results indicate no statistically significant difference in the predicted internal consistency estimate for I-E Scale versus NSIE Scale scores. Only the percentage female variable is found to predict variation in internal consistency estimates. Testing interval length explains variability in test-retest coefficient estimates. Results and directions for future research are discussed. (Contains 2 tables.)