Some Issues in the Statistical Analysis of Vehicle Emissions

Some of the issues complicating the statistical analysis of vehicle emissions and the effectiveness of emissions control programs are presented in this article. Issues discussed include: the variability of inter- and intra-vehicle emissions; the skewness of emission distribution from in use vehicles; the difficulty of obtaining statistically representative vehicle samples; the effect of repeated testing on a subset of the vehicle fleet; and, differences among common test methods and pollutant measurement devices. The article discusses the relevance of these issues in light of the following regulatory purposes: compliance testing of in-use vehicles that have certification standards; effectiveness evaluation of inspection and maintenance programs of vehicles; and emissions inventory estimates for air quality modeling and compliance planning. The article also provides a brief history and description of common vehicle emission tests.