Next section: Confidence intervals and hypothesis testing

One-tailed and two-tailed tests have the same Type I error rate. One-tailed tests are sometimes used when the experimenter predicts the direction of the effect in advance. This use of one-tailed tests is questionable because the experimenter can only reject the null hypothesis if the effect is in the predicted direction. If the effect is in the other direction, then the null hypothesis cannot be rejected no matter how strong the effect is. A skeptic might question whether the experimenter would really fail to reject the null hypothesis if the effect were strong enough in the wrong direction. Frequently the most interesting aspect of an effect is that it runs counter to expectations. Therefore, an experimenter who committed himself or herself to ignoring effects in one direction may be forced to choose between ignoring a potentially important finding and using the techniques of statistical inference dishonestly. One-tailed tests are not used frequently. Unless otherwise indicated, a test should be assumed to be two-tailed.

Next section: Confidence intervals and hypothesis testing