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P Values

George W. Brown, MD
Am J Dis Child. 1990;144(4):493-495. doi:10.1001/archpedi.1990.02150280115026.
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The use of P values (eg, P≤.05 or similar notation) is commonplace in research reports in medical journals. There are two kinds of P values of interest to journal readers. One is the value selected by the researcher as the accepted "risk of the type I error," "α risk," or "rejection region." It is a widespread custom to use.05 as a convenient cut-point for "statistical significance." (This convention is a part of the statistical legacy of the British statistician, Sir Ronald A. Fisher.) The second kind of P value is the observed value from a statistical test, that is, the value resulting from application of a test (eg, t or Χ2 test) to observations. Since the "P" in P value implies a probability, most medical journals1 use an uppercase P, reserving a lowercase p to signify proportions. The aim here is to look at some slippery features


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