Evidence-appraisal glossary
Variance
Variance is the average of the squared distances between each value and the mean; it is the square of the standard deviation and measures how spread out a set of values is.
Variance underlies many statistical procedures because squared deviations have convenient mathematical properties, and terms like analysis of variance refer to partitioning it among sources. Because it is in squared units, such as squared millimeters of mercury, it is hard to interpret directly, which is why reports usually convert it back to the standard deviation for readers.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.