def grade_stability(avg_smape):
    """Qualitative label for the average symmetric mean absolute percentage 
    error (SMAPE).
    
    Parameters
    ----------
    avg_smape : float (> 0)
        The average absolute relative difference.
    
    Returns
    -------
    qualitative_label : str
        The qualitative label for the given average SMAPE. Can be: 'poor', 
        'moderate', 'good' or 'excellent'
    """
    
    qualitative_label = None
    
    if avg_smape <= 5.0:
        qualitative_label = 'excellent' 
    elif avg_smape <= 10.0:
        qualitative_label = 'good' 
    elif avg_smape <= 20.0:
        qualitative_label = 'moderate'
    else:
        qualitative_label = 'poor'
    
    return qualitative_label