computes the survival function of a mixture of a chi-squared random variable of degree
        0 and a scaled chi-squared random variable of degree d

def sf(self,lrt):
        """
        computes the survival function of a mixture of a chi-squared random variable of degree
        0 and a scaled chi-squared random variable of degree d
        """
        _lrt = SP.copy(lrt)
        _lrt[lrt<self.tol] = 0
        pv = self.mixture*STATS.chi2.sf(_lrt/self.scale,self.dof)
        return pv