SigHat: estimated model covariance (matrix or scalar)
beta: fixed effects estimates
method: the method for computing r2beta
obsperclust: number of observations per cluster (i.e. subject)
nclusts: number of clusters (i.e. subjects)
Returns
A vector with the Wald statistic (ncp), approximate Wald F statistic (F), numerator degrees of freedom (v1), denominator degrees of freedom (v2), and the specified r squared value (Rsq)