A p value expresses the probability that a given statistical result is due to chance. p Values are automatically produced for many statistical procedures by analysis packages such as SAS, SPSS, and Stata, and they are commonly cited in epidemiological and medical research as evidence that results should be considered significant. For instance, a clinical trial concerning the effect of two different drugs on a medical outcome would almost certainly cite one or more p values as evidence of the influence, or lack thereof, of the drugs on the outcome. In the example above, perhaps we would expect that if the two drugs are no different in their effect, we would expect the outcome to be the same in each group. Therefore, we would ...

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