In accordance with Federal regulations, state and local air pollution control agencies have, since January 1, 1981, been performing special checks of their ambient air measurement systems to assess the precision and accuracy of the monitoring data. The measurement methods involved are those for total suspended particulate, nitrogen dioxide, sulfur dioxide, carbon monoxide, ozone, and lead. The precisions of the measurement systems are estimated by some type of replicate measurement, including the use of collocated samplers. Accuracies of the measurement systems are estimated by measurement of, or comparison with, reference materials or devices. Because the variations of imprecision and inaccuracy are usually proportional to pollution concentration levels the assessments are expressed on a percentage basis. The precision and accuracy assessments are reported to EPA. Annual EPA reports are issued to summarize and evaluate the data quality. Appreciable variation of data quality exists across the regions, states, and local agencies of the nation.