One of several effect size measures to use in the context of an F-test
for ANOVA or multiple regression. Its amount of bias (overestimation of
the effect size for the ANOVA) depends on the bias of its underlying
measurement of variance explained (R², η², ω²)
https://en.wikipedia.org/wiki/Effect_size#Cohen.27s_.C6.922
R²
ƒ² = ------
1 - R²
η²
ƒ² = ------
1 - η²
ω²
ƒ² = ------
1 - ω²