Measurement Uncertainty

Any scientific measurement has some error associated with it. The concept of “measurement uncertainty” means that for even the most carefully performed measurement, the value of the thing being measured can never be known exactly – only an estimated value can be given.

Measurement uncertainty and error rates can come into play in forensic science whenever a numerical measurement is made, for example with blood or breath alcohol levels or even in measurements such as IQ scores.

In recent cases, courts have held that a calculation of uncertainty or error rate must be reported in order for blood alcohol measurements to be admissible. I’ve prepared a website with articles explaining this topic, motions and opinions from recent cases. Challenging the admissibility of a measurement where no error rate is given may seem most relevant in a DWI case where a BAC of .08 is reported, but it can also be used to strengthen other challenges regarding the reliability of forensic evidence. Attorneys should ask the lab prior to trial about the lab’s calculation of error rates and should expect that chemical analysts will be familiar with the concept given the attention it is currently receiving in the forensic community.

Recent reports and model lab standards support the use of error rates. ISO 17025 is an international standard for testing and calibration of laboratories and is the accreditation standard that the State Crime Lab is currently working to achieve. The Drug Chemistry Section of the Crime Lab has said it is already operating under some ISO standards. ISO standard 5.4.6 requires labs to estimate uncertainty of measurements.

Additional support for requiring measurement uncertainty to be calculated comes from the 2009 Strengthening Forensic Science in the United States: A Path Forward which states “[f]ew forensic science methods have developed adequate measures of the accuracy of inferences made by forensic scientists. All results for every forensic science method should indicate the uncertainty in the measurements that are made, and studies must be conducted that enable the estimation of those values.” (See p. 184.)

If you have questions about whether you can challenge measurement uncertainty in your case, let me know!

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