def _to_z_score(scaled_score, expected_score, test):
    """ Turn scaled and expected score to a z score

    :param scaled_score: scaled score, result from raw_to_scaled function
    :param expected_score: expected score, result from get_expected_score function
    :param test: test of interest
    :return: z-score for the test of interest
    """
    denominator_dict = {'sdmt': 2.790,
                        'bvmt': 2.793,
                        'cvlt': 2.801}

    denominator = denominator_dict.get(test)

    z_score = (scaled_score - expected_score)/denominator

    return z_score