When we test a hypothesis, we decide to either reject or fail to reject the null hypothesis. Our decision can be correct or incorrect. Thus, we have four possible outcomes. Two are good outcomes; two involve error.

1. Correctly reject the null hypothesis when it is false.

2. Incorrectly reject the null hypothesis when it is true.

3. Correctly fail to reject the null hypothesis when it is true.

4. Incorrectly fail to reject the null hypothesis when it is false.

We have names for each type of error. Incorrectly rejecting the null hypothesis is known as Type I error. Type I error is also our alpha level in our hypothesis test (the level of error we are willing to accept in our test). We use the Greek letter alpha (α) to represent Type I error. Incorrectly failing to reject the null hypothesis when it is false is known as Type II error. We use the Greek letter beta (β) to represent Type II error.

We can display the different types of error in a table.