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This paper presents an analysis of third order autoregressive time series. The parameter regions, in the three dimensional parameter space, that produce the six separate types of power spectral density are analyzed. The study reveals that when a particular two dimensional cross section of the three dimensional parameter space is taken, the region of stability is always triangular. Within each triangular stability region in this two dimensional space, subregions which produce the six possible types of spectral shape are indicated. From these subregions it is possible to approximately choose the parameters necessary to model a process whose power spectral density contains at most two critical frequencies (maxima and minima).

Emission tests were conducted in Colorado to determine the contribution automobiles make to air pollution and to see if there is any correlation between carbon monoxide (CO) or hydrocarbon (HC) emissions and vehicle characteristics. The study used statistical regression analysis to compare model years, odometer readings, and engine displacements with emissions. Car model year and maintenance standards were found to be significant variables. Since neither HC nor CO emissions can be accurately predicted from the other, it is recommended that both be measured as a way to reduce maintenance costs.