The agreement between BAC and the breath analyzer result was best when suspects were close to the statutory alcohol limit, namely 0.50 g/kg blood or 0.22 mg/L in breath. The large random variation, as reflected in a residual SD of 0.40 g/kg at a mean BAC of 1.72 g/kg, would introduce considerable uncertainty if breath-test results were used to estimate BAC in individual cases. This large residual SD is one consequence of hundreds of different screening instruments in routine use by the police to control sobriety and the care taken with calibration and quality control of the instruments.