function s = getSubjectIDString(model)
    tmp_sidx =  get(model.subjectSelector,'Value');
    tmp_cellList = getSubjectCellList(model.subjectMap);
    s = cell2mat(tmp_cellList(tmp_sidx));
    if ~ischar(s)
        error('spmtoolbox:SVMCrossVal:getSubjectID:NoString','convert error');
    end
end