Two new indices to detect answer copying

Two new indices to detect answer copying on a multiple-choice test—S1 and S2—were proposed. The S1 index is similar to the K index (Holland, 1996) and the K2 index (Sotaridona & Meijer, 2002) but the distribution of the number of matching incorrect answers of the source and the copier is modeled by the Poisson distribution instead of the binomial distribution to improve the detection rate of K and K2. The S2 index was proposed to overcome a limitation of the K and K2 index, namely, their insensitiveness to correct answers copying. The S2 index incorporates the matching correct answers in addition to the matching incorrect answers. A simulation study was conducted to investigate the usefulness of S1 and S2 for 40- and 80-item tests, 100 and 500 sample sizes, and 10%, 20%, 30%, and 40% answer copying. The Type I errors and detection rates of S1 and S2 were compared with those of the K2 and the ω copying index (Wollack, 1997). Results showed that all four indices were able to maintain their Type I errors, with S1 and K2 being slightly conservative compared to S2 and ω. Furthermore, S1 had higher detection rates than K2. The S2 index showed a significant improvement in detection rate compared to K and K2.