Evaluating Vehicle Emissions Inspection and Maintenance Programs (2001) / Chapter Skim
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Appendix C: Some Statistical Issues in Inspection and Maintenance Evaluations
Appendix C: Some Statistical Issues in Inspection and Maintenance Evaluations
Pages 234-242
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From page 234... ...
As discussedin Chapter ~ ofthis report, thereis considerable correlation between exhaust HC and CO emissions in high-emitting vehicles, but little relationship between these and high NOX emissions. High evaporative emissions may or may not be correlated with high exhaust emissions (Pierson et al.
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, and by auto manufacturers. Responses to mail solicitations have shown low voluntary acceptance by vehicle owners; CARB and EPA have typically experienced acceptance rates on the order of 10°/0.
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Vehicles that are unsafe to test on a dynamometer would not be tested in an I/M program using loaded-mode testing.3 FACTORS INFLUENCING VEHICLE EMISSIONS Vehicle emissions are influenced by numerous factors other than I/M. When evaluating the effect of an I/M program, other factors that may be 2In an earlier Roadside Testing program in California, remote sensing showed that vehicles whose owners refused to allow testing had emissions more than twice those whose owners agreed to the roadside tests (Stedman et al.
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can be expected to have faster rates of deterioration than similar vehicles driven fewer miles. Another important factor influencing vehicle emissions is driving mode (i.e., cold start, warm start, low load, high load, high acceleration or deceleration)
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The use of roadside pullover testing and/or remote-sensing measurements is not sensitive to test fraud and could help to identify testing stations that should be subject to covert audits to detect fraudulent behavior. NUMBER OF VEHICLES I/M programs are designed to minimize the percentage of high-emitting vehicles.
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To keep the frequency ofthese vehicles in perspective, aparalle] record ofthe frequency ofthe fleet segment in the total fleet has to be obtained.
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Assumptions about the lack of influence of certain factors should be checked with a null hypothesis. CONFIDENCE LIMITS Confidence limits of vehicle emissions in log or gamma distributions are asymmetric and can be generated using bootstrap analysis.
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Significant differences between fleets may be caused by differences in any and all ofthe following: vehicle emission control and fuel system technologies, vehicle ages, vehicle types, inspection maintenance histories, socioeconomic owner histories. In addition, similar filet and environmental conditions may be required (altitude, temperature, etc.)
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From page 242... ...
242 Evaluating Vehicle Emissions I/M Programs ditions for the test, reference, and environmental fleet emission measurements. However, fuel and environmental conditions are not taken into account in the pass/fai!
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