statistical hypothesis test one tailed test a one-tailed test is a statistical test which, assuming an unskewed probability distribution, allocates all of the significance level to evaluate only one hypothesis to explain a difference. The one-tailed test provides more power to detect an effect in one direction by not testing the effect in the other direction. one-tailed test should be preceded by two-tailed test in order to avoid missing out on detecting alternate effect explaining an observed difference.