def create_concept_cd(concept_cd_series, join_char='+', exclude_study_level
    =True):
    """Turn the pandas series object into a concept_cd string."""
    concept_cd_series = concept_cd_series.dropna()
    if exclude_study_level:
        concept_cd_series = concept_cd_series[1:]
    concept_code = join_char.join(concept_cd_series)
    return concept_code
