A Continuous a-Stratification Index for Item Exposure Control in Computerized Adaptive Testing

The method of (Formula presented.) -stratification aims to reduce item overexposure in computerized adaptive testing, as items that are administered at very high rates may threaten the validity of test scores. In existing methods of (Formula presented.) -stratification, the item bank is partitioned into a fixed number of nonoverlapping strata according to the items’ (Formula presented.), or discrimination, parameters. This article introduces a continuous (Formula presented.) -stratification index which incorporates exposure control into the item selection index itself and thus eliminates the need for fixed discrete strata. The new continuous (Formula presented.) -stratification index is compared with existing stratification methods via simulation studies in terms of ability estimation bias, mean squared error, and control of item exposure rates.