'Returns the residuals centered within each group.  The
residuals are returned as a list of arrays containing the
centered residuals for each cluster.'

@cache_readonly
def resid_centered_split(self):

    sresid = []
    for v in self.model.group_labels:
        ii = self.model.group_indices[v]
        sresid.append(self.centered_resid[ii])
    return sresid
