How Days Between Tests Impacts Alternate Forms Reliability in Computerized Adaptive Tests

An essential question when computing test-retest and alternate forms reliability coefficients is how many days there should be between tests. This article uses data from reading and math computerized adaptive tests to explore how the number of days between tests impacts alternate forms reliability coefficients. Results suggest that the highest alternate forms reliability coefficients were obtained when the second test was administered at least 2 to 3 weeks after the first test. Even though reliability coefficients after this amount of time were often similar, results suggested a potential tradeoff in waiting longer to retest as student ability tended to grow with time. These findings indicate that if keeping student ability similar is a concern that the best time to retest is shortly after 3 weeks have passed since the first test. Additional analyses suggested that alternate forms reliability coefficients were lower when tests were shorter and that narrowing the first test ability distribution of examinees also impacted estimates. Results did not appear to be largely impacted by differences in first test average ability, student demographics, or whether the student took the test under standard or extended time. It is suggested that for math and reading tests, like the ones analyzed in this article, the optimal retest interval would be shortly after 3 weeks have passed since the first test.