Going to the Doctor as a Metaphor for Significance Testing, Errors and Power
In statistics, you are usually looking for some result such as:

• A difference between means (as with a t-test or ANOVA).
• A difference between proportions (as with a chi-square).
• A relationship (as with a correlation or multiple regression).

Think of it this way. You go to the doctor because you want to know if you are sick. This is like looking for a statistical difference. However, there are several possible results.

• You might be sick or you might not be sick.
• Independent of this, the doctor may say you are sick or the doctor may tell you that you are not sick.

This leads to four outcomes. Two are correct and two are wrong. The one we are interested in is when you are really sick and the doctor says you are really sick.