Comparison of answer copying indices with real data

This study investigated the Type I error rate and power of four copying indices, K-index (Holland, 1996), Scrutiny! (Assessment Systems Corporation, 1993), g, (Frary, Tideman, & Watts, 1977), and omega (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 omega index demonstrated the best Type I error control and power in all conditions and at all alpha levels. Scrutiny! and the K-index were uniformly conservative, and both had poor power to detect true copiers at the small alpha levels typically used in answer copying detection, whereas g(2) was generally too liberal, particularly at small alpha levels. Some comments on the proper uses of copying indices are provided.