def _calc_delta(self,ensemble,scaling_matrix=None):
        '''
        calc the scaled  ensemble differences from the mean
        '''
        mean = np.array(ensemble.mean(axis=0))
        delta = ensemble.as_pyemu_matrix()
        for i in range(ensemble.shape[0]):
            delta.x[i,:] -= mean
        if scaling_matrix is not None:
            delta = scaling_matrix * delta.T
        delta *= (1.0 / np.sqrt(float(ensemble.shape[0] - 1.0)))
        return delta
calc the scaled  ensemble differences from the mean