### Statistical power

Statistical power measures the extent to which you can reject the null hypothesis and is a very important calculation to support the validity of your research project.

Power is highly dependent on your sample size but another element of a power calculation is the effect size, which can be measured using the Cohen’s d statistic. The d value will split the effect being measured into categories:

- Negligible
- Small
- Medium
- Large

The sample size, effect size and power make up a statistical power calculation. A power calculator can take two of those measurements to calculate the third which is useful in planning your study e.g. to calculate the sample size you require to achieve acceptable power. 80% (0.8) is the standard guideline for acceptable power.