Metrology is the science of measurement. The State often relies upon measurements to prove allegations against our clients. For example, the determination of breath and blood alcohol concentrations in a DWI trial involve measurement. The weight of a controlled substance, and its identity, involves measurement. Other examples may include DNA and radar speed detection. Essentially, a forensic science implicating physics, chemistry, toxicology, engineering, psychology, or medicine may also implicate the science of measurement. When we encounter these measurements in the courtroom, we must be prepared to confront them. Consequently, a special understanding of metrology may help us deconstruct a measurement relied upon by the prosecutor. Our ability to discredit a given measurement could be the key to creating reasonable doubt.
Defense lawyers encountering forensic measurements in the courtroom are too often overwhelmed by their attempt to develop a separate expertise in several distinct measurement disciplines. Even the most dedicated defense attorney could be dumbfounded by trying to learn varied technologies spanning diverse fields. But our most grievous mistake would be to acquiescence without a fight. So what can we learn from metrology to help us critically evaluate any forensic measurement? That is, regardless of the discipline, how can we analyze a measurement to determine whether the evidence presented is scientifically sound? There are three metrological components that should be considered any time we evaluate a forensic measurement. These pertain to traceability, calibration, and uncertainty.
Traceability involves the property of a measurement where the result is linked to a known reference via a documented and unbroken chain of comparisons. This anchors the quantitative result to the known reference. Without traceability we cannot be confident in the correctness of the quantitative result reported. In other words, proper traceability helps us determine the measured result is what it purports to be. For example, an alcohol breath test result could not be considered reliable if the traceability of the reference solution was not established. In the United States, national standards for weights and measures are maintained by the National Institute of Standards and Technology (NIST). Once a lawyer understands the traceable components of a given measurement, they can prepare to challenge whether the proof offered is traceable to a known reference. Again, this is accomplished through a documented and unbroken chain of comparisons with the NIST standard.
Calibration is the comparison of measured values created by a device with those values of a calibration standard of known accuracy. The calibration standard is normally traceable to a national standard held by NIST. This process determines how our measuring system responds to quantities with different values so responses generated during later measurements can be charted into correct quantitative values. For example, by determining how a gas chromatograph responds to known and varying alcohol concentrations, the machine’s response to an unknown blood concentration can be confidently mapped to the true concentration. Without proper calibration we cannot be confident the values obtained by a measuring device correspond to those values that could reasonably be attributed to the measuring system. To this end, every measuring device must be calibrated prior to use. It must also be calibrated over the intended range of measurement and be re-calibrated on a regular basis.
Lastly, measurement uncertainty relates to the range of values attributed to a single measured quantity. Uncertainty is inherent in every scientific process. It’s important because no measurement can ever tell us what a quantity’s true value is. In the best case, the measuring system provides a range of values that has a known probability of containing the quantity’s value. For example, the current Austin Crime Laboratory policy for reporting blood alcohol concentrations is with a 99.7% confidence level. The BAC is reported ±9.4%. In other words, crime laboratory chemists reporting a 0.291 result with a 99.7% confidence level would testify they were 99.7% sure the true BAC value lay between 0.263 and 0.319. Without stating measurement uncertainty, any conclusion based upon a measured result is speculation since there’s no way to understand what the result actually represents.
With a basic understanding of metrology, criminal defense lawyers can conduct a critical analysis of forensic measurements regardless of the discipline. Without developing an expertise in varied technologies spanning diverse fields, they can better understand evidence from forensic measurements, they can better prepare and present cases involving such evidence, and they can better recognized poor measurement practices.