In the example, the one-tailed probability would be the probability of obtaining a value of M

The shaded area in the figure is greater than five. The figure shows that the one-tailed probability is 0.036.

It is easier to reject the null hypothesis with a one-tailed than with a two-tailed test as long as the effect is in the specified direction. Therefore, one-tailed tests have lower Type II error rates and more power than do two-tailed tests. In this example, the one-tailed probability (0.036) is below the conventional significance level of 0.05 whereas the two-tailed probability (0.072) is not. Probability values for one-tailed tests are one half the value for two-tailed tests as long as the effect is in the specified direction.