Estimating conditional standard errors of measurement for tests composed of testlets

The primary purposes of this study were to investigate both the appropriateness and the implications of incorporating a testlet definition into the estimation of the conditional standard error of measurement (SEM) for tests composed of testlets. When individual items are used as the fundamental measurement unit with tests composed of testlets, the assumptions required by measurement modeling are violated, but those assumptions are satisfied when testlet scores are used as the measurement unit. Therefore, item-based estimation methods probably introduce some magnitude of bias in the estimates of 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.