Testing the Untestable Assumptions of the Chain and Poststratification Equating Methods for the NEAT Design. Research Report. ETS RR-06-17

This paper focuses on the Non-Equivalent Groups with Anchor Test (NEAT) design for test equating and on two classes of observed--score equating (OSE) methods--chain equating (CE) and poststratification equating (PSE). These two classes of methods reflect two distinctly different ways of using the information provided by the anchor test for computing OSE functions. Each of the two classes includes linear and nonlinear equating methods. In practical situations, it is known that the PSE and CE methods tend to give different results when the two groups of examinees differ in ability. However, given that both methods are justified by making untestable assumptions, it is difficult to conclude which, if either, of the two equating approaches is more correct. This study compares predictions from both the PSE and the CE assumptions that can be tested in a comparable way with the data from a special study. Results indicate that both CE and PSE make very similar predictions but that those of CE are slightly more accurate than those of PSE.