**Type II errors** (**beta errors, β**) occur when we accept that there is no difference between two experimental groups, when in fact, there is a difference.

The p-value does not give a direct indication of the likelihood of a type II error; if the p-value is >0.05, this does not necessarily mean that the two experimental groups are similar. This would only be true if the study is powered to a size to detect a difference if it were present ^{2}.

By convention, we try to keep β = 0.15-0.20

Factors that affect the β:

- the size of the difference between the groups
- variability within groups
- the likelihood of committing a type I error
- sample size: the larger study, the more power and the less likelihood of a β error