sphericity

(noun)

A statistical assumption requiring that the variances for each set of difference scores are equal.

Related Terms

  • order effect
  • longitudinal study

Examples of sphericity in the following topics:

  • Within-Subjects ANOVA

    • Discuss courses of action that can be taken if sphericity is violated
    • Naturally the assumption of sphericity, like all assumptions, refers to populations not samples.
    • Although ANOVA is robust to most violations of its assumptions, the assumption of sphericity is an exception: Violating the assumption of sphericity leads to a substantial increase in the Type I error rate.
    • Possible violation of sphericity does make a difference in the interpretation of the analysis shown in Table 2.
    • A final method for dealing with violations of sphericity is to use a multivariate approach to within-subjects variables.
  • Repeated Measures Design

    • Sphericity: Difference scores computed between two levels of a within-subjects factor must have the same variance for the comparison of any two levels.
    • This test is not recommended for use when there are more than 2 levels of the within-subjects factor because the assumption of sphericity is commonly violated in such cases.
    • Alternative Univariate test: These tests account for violations to the assumption of sphericity, and can be used when the within-subjects factor exceeds 2 levels.
    • Multivariate Test: This test does not assume sphericity, but is also highly conservative.
    • The rANOVA is still highly vulnerable to effects from missing values, imputation, unequivalent time points between subjects, and violations of sphericity.
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