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The Statistician's Page

What is the Value of a p Value?

Medical Data Research Center, Providence Health & Services, Portland, Oregon

^_* Address correspondence to Dr Wu, 9205 SW Barnes Rd, Ste 33, Portland, OR 97225 (Email: yingxing.wu{at}providence.org).

Successful publication of a research study usually requires a small p value, typically p < 0.05. Many clinicians believe that a p value represents the probability that the null hypothesis is true, so that a small p value means the null hypothesis must be false. In fact, the p value provides very weak evidence against the null hypothesis, and the probability that the null hypothesis is true is usually much greater than the p value would suggest. Moreover, even considering "the probability that the null hypothesis is true" is not possible with the usual statistical setup and requires a different (Bayesian) statistical approach. We describe the Bayesian approach using a well-established diagnostic testing analogy. Then, as a practical example, we compare the p-value result of a study of aprotinin-associated operative mortality with the more illuminative interpretation of the same study data using a Bayesian approach.

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