So let's think about that a second. I'll go back to my z-table. We want to have a significance level of 5%, which means the entire area of our rejection, the entire area in which we would reject the null hypothesis, is 5%. This is a two-tailed test. An extreme event either far above the mean or far below the mean will allow us to reject the hypothesis. So we care about area over here. And over here we would put 2.5%.