Rasch fit statistics as a test of the invariance of item parameter estimates

The invariance of the estimated parameters across variation in the incidental parameters of a sample is one of the most important properties of Rasch measurement models. This is the property that allows the equating of test forms and the use of computer adaptive testing. It necessarily follows that in Rasch models if the data fit the model, than the estimation of the parameter of interest must be invariant across sub-samples of the items or persons. This study investigates the degree to which the INFIT and OUTFIT item fit statistics in WINSTEPS detect violations of the invariance property of Rasch measurement models. The test in this study is a 80 item multiple-choice test used to assess mathematics competency. The WINSTEPS analysis of the dichotomous results, based on a sample of 2000 from a very large number of students who took the exam, indicated that only 7 of the 80 items misfit using the 1.3 mean square criteria advocated by Linacre and Wright. Subsequent calibration of separate samples of 1,000 students from the upper and lower third of the person raw score distribution, followed by a t-test comparison of the item calibrations, indicated that the item difficulties for 60 of the 80 items were more than 2 standard errors apart. The separate calibration t-values ranged from +21.00 to -7.00 with the t-test value of 41 of the 80 comparisons either larger than +5 or smaller than -5. Clearly these data do not exhibit the invariance of the item parameters expected if the data fit the model. Yet the INFIT and OUTFIT mean squares are completely insensitive to the lack of invariance in the item parameters. If the OUTFIT ZSTD from WINSTEPS was used with a critical value of | t | > 2.0, then 56 of the 60 items identified by the separate calibration t-test would be identified as misfitting. A fourth measure of misfit, the between ability-group item fit statistic identified 69 items as misfitting when a critical value of t > 2.0 was used. Clearly relying solely on the INFIT and OUTFIT mean squares in WINSETPS to assess the fit of the data to the model would cause one to miss one of the most important threats to the usefulness of the measurement model.