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

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The method of -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 -stratification, the item bank is partitioned into a fixed number of nonoverlapping strata according to the items' , or discrimination, parameters. This article introduces a continuous -stratification index which incorporates exposure control into the item selection index itself and thus eliminates the need for fixed discrete strata. The new continuous -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.