Eta squared (${\eta }^2=S{S}_{\mathrm{effect}}/S{S}_{\mathrm{total}}$)

The fraction of total observed sum of squares attributed to a model effect under a specified analysis of variance decomposition. Its interpretation depends on the design and sum-of-squares convention.

An effect size comparing an effect sum of squares with that effect's own error sum of squares. It need not be the fraction of total observed variation, and partial values for several effects need not sum to one. The relevant error term must be identified, especially in repeated-measures designs.