The reported results of a BAC test in DUI cases appear to provide the answer we are seeking: how much ethanol is in a person’s blood. However, let’s look at the fine print.
The Reported Results
A machine called a gas chromatograph is used to measure an alcohol concentration in blood samples. The measurement, which the machine prints at the end of the process, is called a reported result. When is a reported result a complete measure than can be relied on?
Elements of Complete Measurements
A complete measurement includes more than just a reported result. As a matter of fact, simply providing a reported result is often misleading. A reported result is only complete when accompanied by a “statement of its uncertainty.” See NIST Technical Note 1297, 1994 Edition. No measurement is perfect. The result of any measurement is only an estimation of its value. A “statement of uncertainty” is the range of doubt that exists regarding a measurement.
A complete test result, must also include:
- Reported Result
- “Range of Uncertainty” and;
- “Confidence Interval.”
In addition, to truly understand the reliability and validity of the measurement should also know the error rate of the measurement system.
To illustrate, let’s assume that a blood test result was .100. Let’s also assume, based on a review of the machine’s prior performance, a “range of uncertainty” was determined to be ± 5%, with a “confidence interval” of 100%. This means, the reported result could be as low as a .095 and as high as a .105. Moreover, this also means, if the same blood sample were repeatedly tested on this equipment, the result would only be outside of the ± 5% range 1 out of a million times. If this statistic were true, this would certainty be a reported result that you could trust.
On the other hand, what if for the same reported result of .100 the range was ± 30%, with a confidence interval of 50%? Here, this means the reported result could be as low as .070 or as high as .130. Furthermore, if you continued to test this sample on the same equipment, 300,000 times of out of a million, the reported result would be outside the range stated above.
The Whole Story?
When comparing the two complete test results, you can see that providing a mere reported result does not tell us the whole story. Merely telling us the reported result can actually tell us a very misleading story. Science will not accept incomplete measurements. Why should the law?