| Abstract: | A potential concern for individuals interested in using item response theory (IRT) with achievement test data is that such tests have been specifically designed to measure content areas related to course curriculum and students taking the tests at different points in their coursework may not constitute samples from the same population. In this study, data were obtained from three administrations of two forms of a Biology achievement test. Data from the newer of the two forms were collected at a spring administration, made up of high school sophomores just completing the Biology course, and at a fall administration, made up mostly of seniors who completed their instruction in the course from 6–18 months prior to the test administration. Data from the older form, already on scale, were collected at only a fall administration, where the sample was comparable to the newer form fall sample. IRT and conventional item difficulty parameter estimates for the common items across the two forms were compared for each of the two form/sample combinations. In addition, conventional and IRT score equatings were performed between the new and old forms for each o f the form sample combinations. Widely disparate results were obtained between the equatings based on the two form/sample combinations. Conclusions are drawn about the use o f both classical test theory and IRT in situations such as that studied, and implications o f the results for achievement test validity are also discussed |
