'variance assuming equal variance in both data sets'

@OneTimeProperty
def std_meandiff_pooledvar(self):

    d1 = self.d1
    d2 = self.d2
    var_pooled = ((d1.sumsquares + d2.sumsquares) / (((d1.nobs - 1) + d2.nobs) - 1))
    return np.sqrt((var_pooled * ((1.0 / d1.nobs) + (1.0 / d2.nobs))))
