Comparison of Answer Copying Indices with Real Data

Abstract

This study investigated the Type I error rate and power of four copying indices, K‐index (Holland, 1996), Scrutiny! (Assessment Systems Corporation, 1993), g2 (Frary, Tideman, & Watts, 1977), and ω (Wollack, 1997) using real test data from 20,000 examinees over a 2‐year period. The data were divided into three different test lengths (20, 40, and 80 items) and nine different sample sizes (ranging from 50 to 20,000). Four different amounts of answer copying were simulated (10%, 20%, 30%, and 40% of the items) within each condition. The ω index demonstrated the best Type I error control and power in all conditions and at all α levels. Scrutiny! and the K‐index were uniformly conservative, and both had poor power to detect true copiers at the small α levels typically used in answer copying detection, whereas g2 was generally too liberal, particularly at small α levels. Some comments on the proper uses of copying indices are provided.

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