It is shown that empirical mental test P - P plots are approximately equal to theoretical item-item curves, at least for long tests administered to many people. This result is important because it leads to (1) a distribution free method for estimating points on item-item curves; (2) a general method for defining estimates of item parameters; and (3) the only parameter estimation method known to be consistent for some of the most important aptitude test models. (Author/CTM)