'fixed effects parameters
Display (3.1) of
Laird, Lange, Stram (see help(Mixed)).'

def _compute_a(self):

    for unit in self.units:
        unit.fit(self.a, self.D, self.sigma)
    S = sum([unit.compute_xtwx() for unit in self.units])
    Y = sum([unit.compute_xtwy() for unit in self.units])
    self.Sinv = L.pinv(S)
    self.a = np.dot(self.Sinv, Y)
