Psychometric Consequences of Subpopulation Item Parameter Drift.

This study defines subpopulation item parameter drift (SIPD) as a change in item parameters over time that is dependent on subpopulations of examinees, and hypothesizes that the presence of SIPD in anchor items is associated with bias and/or lack of invariance in three psychometric outcomes. Results show that SIPD in anchor items is associated with a lack of invariance in dimensionality structure of an anchor test, a lack of invariance in scaling coefficients across subpopulations, and a lack of invariance in ability estimates. It is demonstrated that these effects go beyond what can be understood from item parameter drift or differential item functioning.