PRECISION AND DIFFERENTIAL ITEM FUNCTIONING ON A TESTLET-BASED TEST -THE 1991 LAW-SCHOOL ADMISSIONS TEST AS AN EXAMPLE

Two components of the Law School Admissions Test (LSAT)-reading compre hension and analytic reasoning-are not constructed of individual items that can function autonomously. Instead they each consist of four clu sters of items, in which each cluster refers to a common stem. In read ing comprehension the common part is a single passage; in analytic rea soning it is a common situation. Such interdependent clusters of items have come to be called testlets. It has been found that when a test i s constructed of testlets, traditional treatments of individual items as independent entities tends to yield overly optimistic estimates of reliability. It has also been found that even though individual items may pass muster in terms of their differential performance within vari ous subgroups, this may not be true once the items within a testlet ar e treated as a coherent unit. Findings can be divided into three categ ories: (a) overall performance of individual subgroups on the test, (b ) the reliability of each section, and (c) the differential performanc e of items/testlets. This article describes these findings and provide s the details of the methods used in the investigation.