Clinical and experimental pharmacology & physiology
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Clin. Exp. Pharmacol. Physiol. · Jun 2013
Should we use one-sided or two-sided P values in tests of significance?
'P' stands for the probability, ranging in value from 0 to 1, that results from a test of significance. It can also be regarded as the strength of evidence against the statistical null hypothesis (H₀). When H₀ is evaluated by statistical tests based on distributions such as t, normal or Chi-squared, P can be derived from one tail of the distribution (one-sided or one-tailed P), or it can be derived from both tails (two-sided or two-tailed P). ⋯ However, if H₁ is specific and, for example, states than the mean or proportion of Group A is greater than that of Group B, then a one-sided P maybe used. The form that H₁ will take if H₀ is rejected must be stipulated a priori, before the experiment is conducted. It is essential that authors state whether the P values resulting from their tests of significance are one- or two-sided.