Employing components-of-variance to evaluate forensic breath test instruments

Area of Science:

• Forensic Science
• Analytical Chemistry
• Biostatistics

Background:

• Standard evaluations of breath alcohol instruments focus on accuracy and precision.
• These methods often overlook crucial analytical and biological sources of measurement variability.
• Understanding total variance is essential for reliable forensic alcohol analysis.

Purpose of the Study:

• To present an experimental design for assessing breath alcohol instrument suitability.
• To quantify between-instrument effects and subject/instrument interactions.
• To determine the contribution of biological and analytical components to total measurement variance.

Main Methods:

• Three volunteers provided 10 replicate breath samples each into six different instruments.
• Samples were collected within an 18-minute timeframe.
• Two-way analysis of variance (ANOVA) was used to analyze the data.

Main Results:

• Significant differences were found between instruments.
• A significant subject/instrument interaction effect was observed.
• The biological component of total variance ranged widely from 56% to 98%.

Conclusions:

• The proposed experimental design effectively quantifies sources of variance in breath alcohol measurements.
• Biological factors play a substantial role in measurement variability.
• Optimizing breath sampling parameters can reduce variability and improve forensic confidence.