(Received 30 May 1989; accepted 22 November 1989)
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Many jurisdictions have “per se” driving-while-intoxicated (DWI) statutes expressed in terms of a blood-alcohol concentration (BAC) standard (in grams per 100 mL or the equivalent). Since breath-alcohol (BrAC) analysis is typically employed to determine BAC, there is often challenge to the use of an assumed 2100:1 conversion ratio. This concern may be relevant in light of considerable data that show a low percentage of cases in which BrAC > BAC, and this concern increases when the BrAC is used to predict BAC in the context of “per se” legislation.
Probability theory provides a basis for estimating the likelihood of an individual having a BrAC ≥ 0.10 g/210 L with a corresponding BAC < 0.10 g/100 mL. Actual field data from the state of Wisconsin (n = 404) were evaluated to determine the probability of this occurrence. The probability for this occurrence involves the multiplication law for independent events. The computed probability from the data was 0.018. The actual number of occurrences where BrAC ≥ 0.10 g/210 L and BAC < 0.10 g/100 mL was 5, resulting in a probability of 0.012. The concern of having BrAC > BAC at the critical “per se” level has a very low probability of occurrence, which thus supports the reasonableness of “per se” DWI legislation based upon a blood-alcohol standard determined by breath-alcohol analysis.
Breath test technical supervisor, Breath Test Section, Washington State Patrol, Seattle, WA
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