def get_scale(goal_df, norm, results):
    """Get normalization scale factor.

    Parameters
    ----------
    goal_df : pandas.DataFrame
        Clinical goal specifications.
    norm : (str, float, float)
        Region of interest, dose, and volume used for normalization.
    results : dict
        Clinical goal results for current iteration.

    Returns
    -------
    float
        Normalization scale factor.

    """
    index = goal_df.index[(goal_df['Roi'] == norm[0]) &
                          (goal_df['AcceptanceLevel'] == norm[1]) &
                          (goal_df['ParameterValue'] == norm[2])].tolist()[0]
    return norm[1]/results[index]