Eta squared measures the proportion of the total variance in a dependent variable that is associated with the membership of different groups defined by an independent variable. Partial eta squared is a similar measure in which the effects of other independent variables and interactions are partialled out. The development of these measures is described and their characteristics compared. In the past, the two measures have been confused in the research literature, partly because of a labelling error in the output produced by certain versions of the statistical package SPSS. Nowadays, partial eta squared is overwhelmingly cited as a measure of effect size in the educational research literature. Although there are good reasons for this, the interpretation of both measures needs to be undertaken with care. The paper concludes with a summary of the key characteristics of eta squared and partial eta squared. (Contains 3 tables.)