A z-table and critical values are used in hypothesis testing in statistics. Critical values help you to decide when to accept or reject a null hypothesis. Finding a critical value for a right-tailed test will have a familiar feel to it if you are used to looking up areas in a z-table (which should have been covered by this point in any elementary statistics class). Even if you aren’t familiar with using a z-table, this article walks you through the process, step-by-step, with images.
Sample question: Find a critical value in the z-table for an aplha level of 0.0079.
Step 1: Draw a diagram, like the one above. Shade in the area in the right tail.
This area represents α. A diagram helps you to visualize what area you are looking for (i.e. if you want an area to the right of the mean or the left of the mean.
Step 2: Subtract alpha (α) from 0.5.
0.5-0.0079 = 0.4921.
Step 3: Find the result from step 2 in the center part of the z-table.
The closest area to 0.4921 is 0.4922 at z=2.42.