def cdr_to_stage(score):
    """
    Convert the Clinical Dementia Rating scale (CDR) to descriptive terms (O'Bryant et al., 2010).
    This can be helpful for qualitative purposes.

    """
    if score == 0.0:
        return ('Normal')
    elif score == 0.5:
            return ('Questionable')
    elif score == 1.0:
        return ('Mild')
    elif score == 2.0:
        return ('Moderate')
    elif score == 3.0:
        return ('Severe')
    else:
        return ('NaN')