Editing Effect size (section)

===== Eta-squared (''η''<sup>2</sup>) =====
Eta-squared describes the ratio of variance explained in the dependent variable by a predictor while controlling for other predictors, making it analogous to the ''r''<sup>2</sup>. Eta-squared is a biased estimator of the variance explained by the model in the population (it estimates only the effect size in the sample). This estimate shares the weakness with ''r''<sup>2</sup> that each additional variable will automatically increase the value of ''η''<sup>2</sup>. In addition, it measures the variance explained of the sample, not the population, meaning that it will always overestimate the effect size, although the bias grows smaller as the sample grows larger.
<math display="block"> \eta ^2 = \frac{SS_\text{Treatment}}{SS_\text{Total}} .</math>