Which statement correctly defines a Type I error?

Which statement correctly defines a Type I error?

• A. Type I error occurs when you reject H0 when H0 is true.
• B. Type I error occurs when you fail to reject H0 when H0 is true.
• C. Type I error occurs when you reject H0 when H0 is false.
• D. Type I error occurs when you fail to reject H0 when H0 is false.

Answer: (A)

Type I error is a false positive in hypothesis testing: you conclude there is an effect (you reject the null hypothesis) even though the null hypothesis is actually true. This is the scenario described by rejecting H0 when H0 is true. The chance of making this error is set by the significance level, alpha. In contrast, rejecting H0 when H0 is false would be a correct decision, and failing to reject H0 when H0 is true is also a correct decision. Failing to reject H0 when H0 is false is a Type II error.

Type I error is a false positive in hypothesis testing: you conclude there is an effect (you reject the null hypothesis) even though the null hypothesis is actually true. This is the scenario described by rejecting H0 when H0 is true. The chance of making this error is set by the significance level, alpha.

In contrast, rejecting H0 when H0 is false would be a correct decision, and failing to reject H0 when H0 is true is also a correct decision. Failing to reject H0 when H0 is false is a Type II error.