Evidence-appraisal glossary
Statistical power
Statistical power is the probability that a study will detect an effect of a specified size when that effect truly exists; it equals one minus the type II error rate.
Also called: power.
Investigators calculate power before a study to choose a sample size large enough to have a good chance, often 80 or 90 percent, of finding an effect of the size they care about. An underpowered study not only misses real effects but also tends to exaggerate the ones it does flag as significant, so a non-significant result from a small study is weak evidence of no effect rather than proof of none.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.