Use the power curve to help you choose the appropriate sample size or power for your test. This power curve plots the power of the test versus the comparison rate.

If you provide the comparison rate that you want to be able to detect and the size of your sample, Minitab will calculate the power of the test.

For the post office data, the power curves represents every combination of power and comparison rate the test can detect  based on a sample of size of 30, 40, or 50 days. The symbols on the power curves represents the calculated power values.