Estimating conditional standard errors of measurement for tests composed of testlets

The primary purposes of this study were to investigate both the appropriate ness and the implications of incorporating a testlet definition into the es timation of the conditional standard error of measurement (SEM) for tests c omposed of testlets. When individual items are used as the fundamental meas urement unit with tests composed of testlets, the assumptions required by m easurement modeling are violated, but those assumptions are satisfied when testlet scores are used as the measurement unit. Therefore, item-based esti mation methods probably introduce some magnitude of bias in the estimates o f conditional SEMs for tests composed of testlets. The five conditional SEM estimation methods used in this study were classified as either item-based or testlet-based methods. In general, the item-based methods provide lower estimates of the conditional SEM along the score scale than do the testlet -based methods.