||In the spring and fall of 1983, the Quality Assurance Division conducted the National Audits for Stationary Source Test Methods. The audit materials consisted of: a calibrated orifice for Method 5 (dry gas meter only), five simulated liquid samples each for Method 6 (SO2) and Method 7 (NOx), two coal samples for Method 19A, and a disposable gas cylinder for Method 3 (Orsat analyzer). 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 5 spring audit, the mean value for all participants differed by 5.7% from the true (EPA) value. For the fall audit, the participants' mean was 4.1% from the true value. In the two Method 6 audits, the average mean differed by 3% from the expected value. The average mean in the two Method 7 audits was 15% from the expected value. In the two coal audits, the parameters measured were sulfur, moisture, ash, and Btu. On the average for the sulfur analysis 85% of the participants measured within 10% of the expected value; for Btu 100% of the participants measured within 10% of the expected value. In the two Method 3 audits, each parameter had only one concentration. The means for CO2 were within 10% (spring) and 6% (fall) from the expected value. For both audits, the mean for O2 was less than 1%.