When comparing two tests that measure the same trait, separate comparisons should be made at different levels of the trait. A simple, practical, approximate formula is given for doing this. The adequacy of the approximation is illustrated using data comparing seven nationally known sixth-grade reading tests. (Author/RC)

An upper-asymptote parameter was added to the three-parameter logistic item response model. This four-parameter model was compared to the three-parameter model on four data sets. The fourth parameter increased the likelihood in only two of the four sets. Ability estimates for the students were generally unchanged by the introduction of the fourth…