In a previous evaluation of the urban air quality simulation model RAM using hourly SO2 concentrations measured in 1976 at 13 sites during the Regional Air Pollution Study conducted in St. Louis, Missouri, bias with stability was noted. A speculated cause of maxima from the model being calculated during stable conditions was that numerous small and medium point sources were included in the area source part of the emission inventory. Measured maxima occurred over a variety of conditions from unstable to stable. Five statistics: (1) maximum, (2) second highest, (3) median, (4) mean, and (5) minimum related to concentration (both modeled and measured) were calculated for each direction for each of the 13 sites. Various displays of these statistics are used to explore the validity of the speculation regarding source types. Procedures and displays developed can be used in data checking and quality control of emissions inventory data collection as well as in use of receptor models.