A reported result versus a complete result

 

In DUI cases, a machine called a gas chromatograph is often used to measure an alcohol concentration in a blood sample.   The measurement, which the machine prints at the end of the process, is called a reported result.  We are finally at the point in Arizona, where courts are starting to recognize that merely providing a reported result is not sufficient evidence.  The law is coming to the same realization that science did many years ago: a reported result from a machine is an incomplete measurement.

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:

  • a “Range of Uncertainty” and;
  • “Confidence Interval.” 

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.

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?

 

A Well Written Post on The Source Code Issue

I ran across an excellent post discussing the source code issue.  That is, the fight between criminal defense attorneys and the maker of the breath tester (CMI) to disclose the code used in their breath testers.  CMI will not allow an inspection of the code.  Consequently, it cannot be checked for accuracy.  CMI essentially tells everyone charged with DUI to "just trust us."

Ed Brayton, a journalist and the co-founder of Michigan Citizens for Science, discusses the source code litigation in Florida.

Here's a very interesting case from Florida, where an appeals court has upheld a lower court ruling that threw out evidence from a breathalyzer test in a drunk driving case because the manufacturer of the device refused to release the source code and allow defense experts to analyze the accuracy of the machines.

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Wonder Bread Does it Again!

It turns out my previous post showing Wonder Bread causing a false blood alcohol concentration reading was not an isolated incident.  The same forensic toxicologist and drug recognition expert did another Wonder Bread experiment.  Here it is:

 

Can Bread Cause the Intoxilyzer to Give a False Reading?

As demonstrated below, it appears that Wonder Bread is like Kryptonite to the Intoxilyzer 8000. While I cannot guarantee the accuracy of the test that occurred in this video, I have very little reason to doubt its veracity.  Especially since the manufacture of the Intoxilyzer 8000 (CMI) will not allow anyone (including the government) to inspect the source code used in this contraption.  Enjoy:

 

 

Challenging DUI Breath Testing: The Timing of the Pretest Deprivation Period

Arizona law enforcement often uses breath-testing devices to determine the blood-alcohol concentration (BAC) of a person suspected of DUI.  The Intoxilyzer 8000 is commonly used in Maricopa County.  When a person is suspected of DUI, he is generally requested to blow twice into an Intoxilyzer; this is referred to as “duplicate breath testing.”

The Department of Public Safety (DPS) has issued regulations for duplicate breath testing, which it defines as “two consecutive breath tests that immediately follow a deprivation period, agree within 0.020…of each other...”

In addition, the Department of Public Safety defines a deprivation period as “at least a 15-minute period immediately prior to a duplicate breath test during which period the subject has not ingested any alcoholic beverage or other fluids, eaten, vomited, smoked or placed any foreign object in the mouth.”  Breath-testing experts have stated that the deprivation period is critical to the breath-testing process. (See Kurt Dubowski, “Quality Assurance in Breath-Alcohol Analysis,” Journal of Analytical Toxicology, Vol. 18, October 1994.)
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