Researchers often identify degree effects by including degree attainment ("D") and years of schooling ("S") in a wage model, yet the source of independent variation in these measures is not well understood. We argue that "S" is negatively correlated with ability among degree-holders because the most able graduate the fastest, but positively correlated among dropouts because the most able benefit from increased schooling. Using NLSY79 data, we find support for this argument; our findings also suggest that highest grade completed is the preferred measure of "S" for dropouts, while age at school exit is a more informative measure for degree-holders. (Contains 3 figures, 6 tables, and 15 footnotes.)