Greenhouse geissercorrection is the method of adjusting the degrees of freedom of the within- subject F-tests in the analysis of the variance of longitudinal data so as to allow the possible departures of the variance-covariance matrix of the measurements from the suppositions of sphericity. If this condition holds for data then the correction factor is one and the easy F-tests are valid. Departures from the sphericity result in the estimated correction factor less than one, hence reducing the degrees of freedom of the appropriate F-tests.