How is Greenhouse-Geisser correction calculated?

How is Greenhouse-Geisser correction calculated?

Greenhouse-Geisser Correction 002 to F (1.277,6.384) = 12.534, p = . 009 (degrees of freedom are slightly different due to rounding). The correction has elicited a more accurate significance value. It has increased the p-value to compensate for the fact that the test is too liberal when sphericity is violated.

How do you fix a sphericity violation?

Correcting for Violations of Sphericity In all cases the degrees of freedom are reduced based on an estimate of how ‘spherical’ the data are; by reducing the degrees of freedom we make the F-ratio more conservative (i.e. it has to be bigger to be deemed significant).

How is the Greenhouse-Geisser measure of sphericity used to adjust the statistics of your ANOVA?

Repeated measures ANOVA without assuming sphericity Prism can use the method of Greenhouse and Geisser to adjust the results of the repeated measures ANOVA to account for the value of epsilon. The only thing this adjustment does is reduce the number of degrees of freedom, which increases the P value.

How do I report sphericity correction?

If sphericity is violated, report the Greenhouse-Geisser ε and which corrected results you’ll report: “Since sphericity is violated (ε = 0.840), Huyn-Feldt corrected results are reported.”

How do you interpret sphericity?

The degree to which sphericity is present, or not, is represented by a statistic called epsilon (ε). An epsilon of 1 (i.e., ε = 1) indicates that the condition of sphericity is exactly met. The further epsilon decreases below 1 (i.e., ε < 1), the greater the violation of sphericity.

What should you do if the assumption of sphericity is violated?

If, however, the assumption of sphericity is violated, the F-statistic is positively biased rendering it invalid and increasing the risk of a Type I error. To overcome this problem, corrections must be applied to the degrees of freedom (df), such that a valid critical F-value can be obtained.

What happens when sphericity is violated?

The violation of sphericity occurs when it is not the case that the variances of the differences between all combinations of the conditions are equal. If sphericity is violated, then the variance calculations may be distorted, which would result in an F-ratio that is inflated.

What does the Greenhouse-Geisser correction do?

The Greenhouse–Geisser correction is a statistical method of adjusting for lack of sphericity in a repeated measures ANOVA. The correction functions as both an estimate of epsilon (sphericity) and a correction for lack of sphericity.

What does it mean if greenhouse Geisser is significant?

If the p-value is LESS THAN . 05, then researchers have evidence of a statistically significant main effect amongst the observations of the outcome or within-subjects.

Why would you use a greenhouse Geisser correction?

The Greenhouse-Geisser is used to assess the change in a continuous outcome with three or more observations across time or within-subjects. In most cases, the assumption of sphericity is violated for this type of within-subjects analysis and the Greenhouse-Geisser correction is robust to the violation.

What does Greenhouse-Geisser correction do?

How do you know if sphericity is violated?