Statistics Dictionary

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Sphericity is a critical assumption that must be satisfied when conducting a standard analysis of variance on data from a repeated measures experiment.

Sphericity exists when the variance of the difference between scores for any two levels of a repeated measures variable is constant. If the sphericity assumption is violated, the hypothesis test of a repeated measures effect will be positively biased; that is, the researcher will be more likely to make a Type I error (i.e., reject the null hypothesis when it is, in fact, true).

For an easy-to-understand description of sphericity, including techniques for dealing with sphericity, go here.

See also:  Sphericity and Repeated Measures | Repeated Measures Design