Evaluating the Quantity-Quality Trade-off in the Selection of Anchor Items: a Vertical Scaling Approach

When administering tests across grades, vertical scaling is often employed to place scores from different tests on a common overall scale so that test-takers’ progress can be tracked. In order to be able to link the results across grades, however, common items are needed that are included in both test forms. In the literature there seems to be no clear agreement about the ideal number of common items. In line with some scholars, we argue that a greater number of anchor items bear a higher risk of unwanted effects like displacement, item drift, or undesired fit statistics and that having fewer psychometrically well-functioning anchor items can sometimes be more desirable. In order to demonstrate this, a study was conducted that included the administration of a reading-comprehension test to 1,350 test-takers across grades 6 to 8. In employing a step-by-step approach, we found that the paradox of high item drift in test administrations across grades can be mitigated and eventually even be eliminated. At the same time, a positive side effect was an increase in the explanatory power of the empirical data. Moreover, it was found that scaling adjustment can be used to evaluate the effectiveness of a vertical scaling approach and, in certain cases, can lead to more accurate results than the use of calibrated anchor items.