Equating and item banking with the Rasch model

Abstract

This article describes Rasch measurement procedures for equating multiple test forms or calibrating an item bank. The procedures entail (a) selecting an appropriate data collection design, (b) estimating parameters, (c) transforming the parameters from multiple forms to a common scale, and (d) evaluating the quality of the linkage between these forms. Data collection designs include (a) anchor tests, (b) single group, (c) single data set, and (d) equivalent groups. Estimation procedures may involve (a) separate or (b) simultaneous calibration of data from multiple forms. Transformation is typically accomplished using (a) estimation scaling, but may involve (b) parameter anchoring or (c) computing equating constants. Link quality is evaluated using four fit indices: (a) item-within-link, (b) item-between-link, (c) link-within-bank, and (d) form-within-bank. These procedures are illustrated using an anchor test design.