Greenhouse-Geisser and Huynh-Feldt correction (ε)

ε (epsilon) measures how strongly sphericity is violated: 1 means met, the lower bound is 1 / (time points − 1). The correction multiplies both degrees of freedom of the analysis of variance by ε; F stays the same, p becomes larger. Greenhouse-Geisser is the stricter, Huynh-Feldt the somewhat more generous estimate (often chosen when ε is above .75).

Example: F(1.54, 23.16) = 14.85, p < .001: with ε = .77 the degrees of freedom 2 and 30 became 1.54 and 23.16.

Caution: The degrees of freedom are no longer whole numbers; that is usual. With ε near 1 the correction changes little.