Thursday, May 26, 2011

An Odd Ruling on DUI Error Statistics?

An article in the Lansing State Journal [1] begins as follows:
Blood tests in drunken-driving cases statewide will face more scrutiny, experts say, after a Mason County judge ruled that the state crime lab's test results "are not reliable."

In a ruling signed Friday, 79th District Court Judge Peter Wadel refused to admit blood-alcohol results in a drunken-driving case. He said the crime lab — which conducts blood and other forensic tests in cases from around the state — does not report an error rate, or margin of error, along with blood-alcohol results.

Police routinely report a single number for blood-alcohol content in drunken-driving cases. But East Lansing attorney Mike Nichols, who is handling the case in Mason County — which includes the city of Ludington along Lake Michigan — said there are no absolutes in science.

"Everyone says a blood test is so accurate. Well, it's not," Nichols said. "That's what this judge has ruled."

Not including a range of possible results, Nichols said, ignores the uncertainties in the collection, handling, analysis and reporting process.

A blood-alcohol level of 0.08 percent is the threshold in Michigan for being charged with drunken driving. But Nichols said when someone's blood-alcohol is determined to be 0.10, for example, it could actually be higher - or lower - than 0.08.
Mr. Nichols is correct — in part. Because of measurement error in blood alcohol testing, there is some chance that a true concentration of just under 0.08% could give rise to reading of 0.10%. The equipment can be calibrated and shown to have a standard error of measurement. This statistic should be included in laboratory reports, and judges should learn what it means. (The statistics chapter of the Federal Judicial Center's Reference Manual on Scientific Evidence [2] is one of many sources for an explanation.)

But what about all the other possible sources of errors, from mislabeling samples to falsifying data? "Not including a range of possible results, Nichols said, ignores the uncertainties in the collection, handling, analysis and reporting process." Analysis I get. That was the subject of the preceding paragraph. But "collection, handling, and reporting"? These are matters traditionally handled by proof of chain of custody and cross-examination. It would be difficult to report error probabilities for these matters, and no appellate court, as far as I know, ever has required it.


1. Kevin Grasha, DUI Blood Tests Could Face Scrutiny after Judge's Ruling, Lansing State Journal, May 11, 2011

2. David H. Kaye & David A. Freedman, Reference Guide on Statistics, in Reference Manual on Scientific Evidence (Federal Judicial Center 3d ed. 2011)

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