Defining error rates and power for detecting answer copying

A familywise approach is described for evaluating the significance of copyi ng indices designed to hold the Type I error rate constant for each examine e. The empirical Type I error rate and power of two indices, omega (Wollack , 1997) and g(2) (Frary, Tideman, & Watts, 1977), are examined under a vari ety of copying situations. Results indicated that the traditional pairwise approach falsely detected examinees almost three times more often than the nominal alpha level. Familywise Type I error rates were substantially small er, although they also tended to be somewhat inflated at small alpha levels as the percentage of items copied increased. Eliminating the indices detec ting a source from the copier, in situations where the copier was also dete cted from the source, helped control the familywise Type I error rates for all alpha greater than or equal to .001. Lack of Type I error control meant power could not be evaluated for g(2) under any of the simulated familywis e conditions. Familywise power for omega was reasonable when at least 30% o f the items were copied.