The Effect of Anchor Test Construction on Scale Drift

Abstract

In common-item equating the anchor block is generally built to represent a miniature form of the total test in terms of content and statistical specifications. The statistical properties frequently reflect equal mean and spread of item difficulty. Sinharay and Holland (2007) suggested that the requirement for equal spread of difficulty may be too restrictive. They suggested that an anchor test with representative content coverage and equal mean item difficulty but a smaller spread of item difficulty (miditest) may provide the same or better results for equating while decreasing the pressure to find very hard and very easy items to include in the anchor. Analyses to date have concentrated on the results of equating the scores from one form to another with findings that are supportive of the Sinharay and Holland concept (Sinharay & Holland, 2006a, 2006b, 2007; Liu, Sinharay, Holland, Feigenbaum, & Curley, 2009). These studies do not address longer chains of equating. It is important to monitor the possibility of scale drift over forms. The current research begins to address this issue.