A useful combination of results from internal quality control and external quality assessment is possible only if the statistical procedures of both systems involve the same limits to accept control data. These limits are shown. For 45 laboratories expected quality groups QG-T are calculated from results of internal quality control. Quality groups QG-RVL are calculated from results of external quality control assessment. In 30 cases the great difference of QG-T and QG-RVL is supposed to be caused by inaccurate performance of internal quality control.