An inconvenient truth: Legal implications of errors in breath alcohol analysis arising from statistical uncertainty

The general practice in courts throughout Australia is to accept without question the accuracy of what are popularly referred to as 'breathalysers', or breath analysis instruments as they are legally described. The possibility that they might be providing false readings is only considered if that possibility is raised as a matter of evidence by a motorist who has been breathalysed, and who now faces the prospect of legal sanctions as a result of what it is alleged was revealed by the breath analysis instrument. In this article, it is argued that the methodological and statistical bases for such an assumption are unsound, particularly when the blood alcohol level inferred from breath analysis is close to an important, legislatively proscribed, threshold such as 0.05% or 0.15%. It is an inconvenient truth that breath analysis overestimates blood alcohol levels for some individuals at these thresholds, with profound legal consequences.