def _ptid_to_rid(ptid, roster, ptid_label='PTID'):
    """Returns roster id for a given patient id
    ptid_label values : 'PTID', 'SCRNO'
    """

    rid = roster[roster[ptid_label] == ptid]['RID'].values
    if len(rid) > 0:
        return rid[0]
    else:
        return ''