How many digits should be reported in forensic breath alcohol measurements?

Area of Science:

• Analytical Chemistry
• Forensic Science
• Measurement Science

Background:

• Measurement results inherently contain uncertainty.
• The number of reported digits should reflect this uncertainty.
• Current practices in breath alcohol analysis may not be optimal.

Purpose of the Study:

• To evaluate the appropriate number of digits for reporting breath alcohol measurements.
• To compare the uncertainty associated with two-digit versus three-digit results.
• To advocate for evidence-based reporting practices in forensic analysis.

Main Methods:

• Statistical analysis of measurement uncertainty.
• Comparison of combined uncertainty for two- and three-digit truncated results.
• Review of historical practices in breath alcohol analysis.

Main Results:

• The first significant digit of combined uncertainty in breath alcohol measurement appears in the third decimal place.
• Reporting three digits significantly reduces combined uncertainty compared to two digits.
• For a breath alcohol concentration of 0.089 g/210 L, three-digit results yield a combined uncertainty of 0.0031 g/210 L, versus 0.0042 g/210 L for two-digit results.

Conclusions:

• Three-digit truncated results are statistically more appropriate for breath alcohol measurements.
• Modern digital instrumentation supports more precise reporting than historical analog devices.
• The principles discussed are applicable to other quantitative analytical measurements.