A quality assurance program was incorporated into the National Crop Loss Assessment Network (NCLAN) program, designed to assess the economic impacts of gaseous air pollutants on major agricultural crops in the United States. The quality assurance program developed standardized research and monitoring protocols among sites, and included a range of audit and review procedures. The goal was to quantitatively describe the overall quality of data collected in terms of precision, accuracy, completeness, representativeness, and comparability. Project data quality objectives were valuable for determining the acceptability of data from diverse sites. Standardized protocols ensured data comparability among research sites. Independent on-site audits served to evaluate protocol adherence, and precision and accuracy measurements provided a way to assess data quality, determine data acceptability, and indicate the need for instrument adjustment or repair. (Copyright (c) 1988 Elsevier Applied Science Publishers Ltd.)