Type II error

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 .

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