# Statistical power

 Completion status: this resource is ~50% complete.
 Educational level: this is a tertiary (university) resource.

Statistical power is the likelihood that a statistical test will:

1. return a significant result based on a sample from a population in which there is a real effect.
2. reject the null hypothesis when the alternative hypothesis is true (i.e. that it will not make a Type II error).

Power can range between 0 and 1, with higher values indicating a greater likelihood of detecting an effect.

## What is statistical power?

Statistical power is the probability of correctly rejecting a false H0, i.e., getting a significant result when there is a real difference in the population.

## Desirable power

1. Power ≥ .80 generally considered desirable
2. Power ≥ .60 is typical of studies published in major psychology journals

## Increasing power

Power will be higher when the:

1. effect size is larger
2. sample size is larger
3. critical value is larger

## Estimating power

Statistical power can be calculated prospectively and retrospectively.

If possible, calculate expected power before conducting a study, based on:

1. Estimated N,
2. Critical α,
3. Expected or minimum ES (e.g., from related research)

Report actual power in the results.

## Power calculators

Try searching using terms such as "statistical power calculator" and maybe also the type of test, and you should turn up links to useful pages such as:

## References

1. Cohen, J. (1992). Power primer. Psychological Bulletin, 112, 155-159.