The 27 Percent Rule Revisited

Abstract

The problem of selecting the size of the tails from a sample drawn from a distribution of test scores for item analysis study is discussed. In the past the selected tails were viewed as independent samples. However, this is not the case. The tails are dependent. Given this we find that each tail should contain about 21% of the sample and not the traditional 27%. Use of 27%, however, is not far from optimal.