Item Parameter Drift in a Time-Varying Predictor

The current simulation study examined the effects of Item Parameter Drift (IPD) occurring in a short scale on parameter estimates in multilevel models where scores from a scale were employed as a time-varying predictor to account for outcome scores. Five factors, including three decisions about IPD, were considered for simulation conditions. It was revealed that IPD occurring in a relatively shorter scale led to a substantial increase in the amount of relative bias in parameter estimates. The bias was more prominent in the estimates of level-2 time-varying predictors relative to those of level-1 time-varying predictors. Regarding the decisions about IPD, keeping items exhibiting IPD was more appropriate than removing them based on the results from relative bias of standard errors of estimates. Based on the findings, it can be concluded that removing items exhibiting IPD may lead to an increase of Type II errors due to the underestimation of parameter estimates and overestimation of standard errors. The applied example showed findings consistent with those in the simulation study.