# alpha (α)

In hypothesis testing, the probability of a Type I error, i.e. the probability of concluding incorrectly that a null hypothesis is false.

**Note:** For example, α could be the probability of concluding that an intervention is effective when it has no true effect. If the investigators set α at 0.05, this means that they accept that there is a 5% risk of a Type I error. The α-level is commonly set at 0.01, 0.05 or 0.10.

(Related concept: hypothesis testing)