Partial eta squared (${\eta }_p^2=S{S}_{\mathrm{effect}}/(S{S}_{\mathrm{effect}}+S{S}_{\mathrm{error}})$)

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.