Type I and Type II Errors

A type I error occurs when the null hypothesis is true and it is rejected.

A type II error occurs when the null hypothesis is false and it is accepted.


H0 True False
Accepted Correct decision

Probability = 1 - α
Incorrect decision:

TYPE II ERROR
Rejected TYPE I ERROR

Probability = α

Correct decision


The probability of committing a type I error is equivalent to the significance level, α.

The probability of committing a type II error is generally unknown, but is denoted by β.