Imprecise DWI Tests: Good Enough for Government Work?

Posted On September 07, 2011 by Daniel Koewler

Minnesota analyzes drivers' blood, breath and urine to determine a specific level of alcohol concentration. The manner in which the state reports the results leads us to believe they are very certain that the results are 100% accurate.

This is definitely not the case and is inconsistent with well-established scientific standards.

As with all scientific testing, laboratory analyses conducted by forensic scientists are subject to measurement error. Consider the following situation, based on an example taken from 2009 National Academy of Science Report to Congress:

It's a DWI case in which the Minnesota Bureau of Criminal Apprehension tests a driver's blood using "state of the art| instruments. The lab analyzes the blood three times. The instruments' measurements are 0.08 percent, 0.09 percent, and 0.10 percent.

Minnesota takes the lower of the three measurements, and reports that result â?? 0.08 â?? as a certainty. They even try to claim that by taking the lowest number, they're somehow |helping| the test subject by giving them the benefit of the doubt. This is not only incredibly misleading, but in examples like this, downright wrong. It's the type of sleight of hand that looks good, but is flat out unscientific. Scientifically speaking, the |conclusion| that should be reached in the above example is that the test result is no higher than .07! (0.09 +- .02 for those of you with statistics training, and further explained below).

As the NAS Report says, the variability in the three measurements may arise from the internal components of the instrument, the different times and ways in which the measurements were taken, or a variety of other factors. These measured results must be reported, along with a confidence interval that has a high probability of containing the true blood-alcohol level (e.g., the mean plus or minus two standard deviations). For this illustration, the average is 0.09 percent and the standard deviation is 0.01 percent; therefore, a two-standard-deviation confidence interval (0.07 percent, 0.11 percent) has a high probability of containing the person's true blood-alcohol level.

See, 2009 report to Congress of a Committee of the National Academy of Sciences, National Research Council Committee on Identifying the Needs of the Forensic Science Community, Strengthening Forensic Science in the Unites States: A Path Forward, (2009).

In criminal cases the government has the burden to prove beyond a reasonable doubt the driver's alcohol concentration is over a specific alcohol concentration. When the Intoxilyzer reports an alcohol concentration of .08, juries are left to believe that result is a certainty. It is not.

This becomes even more significant when test results are near .04, .08, .16, .20. The government lab report misleads judges, juries and the Minnesota Department of Public Safety. As a result, drivers are erroneously convicted of drunk driving and may go to jail. They may lose their license to drive and face penalties which include ignition interlock, or vehicle forfeiture and plate impoundment.

Next: Are there established confidence intervals for Minnesota DWI tests? What are they? How can I use them in my case?