get the number of non-zero weighted observations

        Returns
        -------
        nnz_obs : int
            the number of non-zeros weighted observations

def nnz_obs(self):
        """ get the number of non-zero weighted observations

        Returns
        -------
        nnz_obs : int
            the number of non-zeros weighted observations

        """
        nnz = 0
        for w in self.observation_data.weight:
            if w > 0.0:
                nnz += 1
        return nnz