A completely nested design is defined to be unbalanced if the groups corresponding to the levels of some classification variable are not all of the same size. The NESTED procedure can compute unbiased estimates for the variance components in an unbalanced design, but because the sums of squares on which these estimates are based no longer have distributions under a Gaussian model for the data, F tests for the significance of the variance components cannot be computed. PROC NESTED checks to see that the design is balanced. If it is not, a warning to that effect appears in the log, and the columns corresponding to the F tests in the analysis of variance are left blank.