Menu location: Analysis_Parametric_F (Variance Ratio).
This function tests the equality of the variances of two random samples from a normal distribution.
F is the ratio of variances (largest as numerator) from samples of size n1 and n2. Degrees of freedom are n1-1 and n2-1 corresponding to the numerator and denominator sample variances.
Only the upper tail probability need be considered because the larger variance is always used as the numerator in the variance ratio F (Altman, 1991; Armitage and Berry, 1994). In most situations this probability should be doubled to give a two sided test. Analysis of variance can utilise a one sided probability because the numerator and denominator of the variance ratio are predetermined.