Equating Errors and Scale Drift in Linked-Chain IRT Equating with Mixed-Format Tests.

In linked-chain equating, equating errors may accumulate and cause scale drift. This simulation study extends the investigation on scale drift in linked-chain equating to mixed-format test. Specifically, the impact of equating method and the characteristics of anchor test and equating chain on equating errors and scale drift in IRT true score equating is examined. To evaluate equating results, a new method is used to derive true linking coefficients. The results indicate that the characteristic curve methods produce more accurate and reliable equating results than the moment methods. Although using more anchor items or an anchor test configuration with more IRT parameters can lower the variability of equating results, neither of them help control equating bias. Additionally, scale drift increases when an equating chain runs longer or poorly calibrated test forms are added to the chain. The role of calibration precision in evaluating equating results is highlighted.