The Alpha Percentage and Experimentwise Error Rates in Communication Research

Experimentwise error rates of the type proposed by Ryan (1959) are discussed and contrasted with anew measure of the likelihood that the results of a series of significance tests are Type I errors. This new measure, the Alpha Percentage (a%), shares the advantages of experimentwise error rates over individual alpha levels in reducing Type I errors in communication research, but the Alpha Percentage has much greater power than currently used experimentwise error rates to detect significant effects. Four arguments against the use of experimentwise error procedures are discussed and EW, EP, and a% rates are reported for Communication Monographs and Human Communication Research.