Alpha level and the critical region
The alpha level is setbefore (at the beginning of an experiment) the analysis of the data.
Use the following Distributions tool to identify the boundaries that separate the extreme samples from the samples that are more obviously consistent with the null hypothesis. Assume the null hypothesis is nondirectional, meaning that the critical region is split across both tails of the distribution.
The z-score boundaries at an alpha level α = .05 are:

z = 3.29 and z = –3.29
z = 2.58 and z = –2.58
z = 1.96 and z = –1.96
To use the tool to identify the z-score boundaries, click on the icon with two orange lines, and slide the orange lines until the area in the critical region equals the alpha level. Remember that the probability will need to be split between the two tails.
To use the tool to help you evaluate the hypothesis, click on the icon with the purple line, place the two orange lines on the critical values, and then place the purple line on the z statistic.