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Cohen 1994 - The Earth Is Round(p <.05 Jacob Cohen After 4...

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The Earth Is Round (p < .05) Jacob Cohen After 4 decades of severe criticism, the ritual of null hy- pothesis significance testing-mechanical dichotomous decisions around a sacred .05 criterion-still persists. This article reviews the problems with this practice, including its near-universal misinterpretation of p as the probability that H o isfalse, the misinterpretation that its complement is the probability of successful replication, and the mis- taken assumption that if one rejects H o one thereby affirms the theory that led to the test. Exploratory data analysis and the use of graphic methods, a steady improvement in and a movement toward standardization in measurement, an emphasis on estimating effect sizes using confidence intervals, and the informed use of available statistical methods is suggested. For generalization, psychologists mustfinally rely, as has been done in all the older sciences, on replication. I make no pretense of the originality of my remarks in this article. One of the few things we, as psychol- ogists, have learned from over a century of scientific study is that at age three score and 10, originality is not to be expected. David Bakan said back in 1966 that his claim that "a great deal of mischief has been associated" with the test of significance "is hardly original," that it is "what 'everybody knows,''' and that "to say it 'out loud' is ... to assume the role of the child who pointed out that the emperor was really outfitted in his under- wear" (p. 423). If it was hardly original in 1966, it can hardly be original now. Yet this naked emperor has been shamelessly running around for a long time. Like many men my age, I mostly grouse. My ha- rangue today is on testing for statistical significance, about which Bill Rozeboom (1960) wrote 33 years ago, "The statistical folkways of a more primitive past continue to dominate the local scene" (p. 417). And today, they continue to continue. And we, as teachers, consultants, authors, and otherwise perpetrators of quantitative methods, are responsible for the rituali- zation of null hypothesis significance testing (NHST; I resisted the temptation to call it statistical hypothesis in- ference testing) to the point of meaninglessness and be- yond. I argue herein that NHST has not only failed to support the advance of psychology as a science but also has seriously impeded it. Consider the following: A colleague approaches me with a statistical problem. He believes that a generally rare disease does not exist at all in a given population, hence H o : P = O. He draws a more or less random sample of 30 cases from this population and finds that one of the cases has the disease, hence P s = 1/30 = .033. He is not December 1994 • American Psychologist Copyright 1994 by the American Psychologicat Association. Inc. 0003-066X/94/$2.oo VoL 49.
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