is a statistical method of adjusting for lack of sphericity in a repeated measures ANOVA.
The correction was proposed by Samuel Greenhouse and Seymour Geisser in 1959.
If sphericity is not met, then epsilon will be less than 1 (and the degrees of freedom will be overestimated and the F-value will be inflated).
[2] To correct for this inflation, multiply the Greenhouse–Geisser estimate of epsilon to the degrees of freedom used to calculate the F critical value.
As a general rule of thumb, the Greenhouse–Geisser correction is the preferred correction method when the epsilon estimate is below 0.75.