Errors

Written by:

Ylva B Almquist

There are two types of error that may occur in hypothesis testing: They are, logically, called type I error and type II errors.

Type I error

A type I error occurs when the null hypothesis is rejected despite being true.

Type II error

A type II error occurs when the null hypothesis is not rejected despite being false.

		Conclusion
		Reject H₀ in favor of H₁	Do not reject H₀
“Truth”	H₀	Type I error	Right decision
	H₁	Right decision	Type II error

Example
In the example of cats and dogs, a type I error would thus occur if we concluded that there is a difference in the intelligence between cats and dogs although that is not true. A type II error, on the other hand, would occur if we concluded that there is no difference in intelligence when in fact there is.

Note
Type I errors are generally considered to be more serious that type II errors. Type II errors are often due to poor statistical power (often because of small sample size).