Abstract
Despite many advances in test theory, estimating measurement error which arises from temporary changes in the person or test situation has re mained relatively unchanged. Unfortunately, not only are short-term instabilities the most important source of measurement error for many traits, especially those measured from highly speeded tests; but the classical test-retest formula for es timating error is based on untenable assumptions with respect to practice effects. The current paper presents a method which gives maximum likelihood estimates of measurement error within the context of a simplex model for practice effects. The appropri ateness of the model is tested for five traits, and error estimates are compared to the classical for mula estimates.
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