In 1988, the Quality Assurance Division conducted the National Audits for Stationary Source Test Methods. The audit materials consisted of: a disposable gas cylinder for Method 3 (Orsat analyzer), a calibrated orifice for Method 5 (DGM only), five simulated liquid samples each for Method 6 (SO2) and Method 7 (NOx), and two coal samples for Method 19. Participating laboratories sent their data to the Source Branch and in return received a written report comparing their results to EPA's. In the Method 3 audit, each parameter had only one concentration. The mean for CO2 was 4.2% from the expected (EPA) value. The mean for O2 was 0.4%. In the Method 5 audit, the mean value for all participants differed by 3.1% from the expected value. For the Method 6 audit, the average mean differed by 3.9% from the expected value, and in the Method 7 audit, the average mean differed by 23.7%. In the two coal audits, the parameters measured were sulfur, moisture, ash, and Btu content. On the average, for the sulfur analysis, 91% of the participants measured within 10% of the expected value; for Btu, 97% of the participants measured within 10% of the expected value.