DUI & Criminal Defense

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DUI LIBRARY | Uncertainty of Measurement

CHAPTER 7:

Uncertainty of Measurement

The primary area of scientific expertise in DUI and vehicular crimes cases is perhaps the what is most lacking our country’s crime laboratories – metrology.

  • A complete measurement 
  • The range of uncertainty
  • The confidence interval of a measurement

A Complete Measurement Requires Showing Its Uncertainty

No measurement can ever claim to represent a true value.  The best that science can do is to provide a value believed to be within a range of values. As stated above, the more important the purpose of the measurement, the more rigor is needed in determining the parameters of that range and confidence that the true value falls within it. These variables are called uncertainty of measurement.

The measurement, which the machine prints at the end of its process, is known as a reported result. The reported result is a purported measurement. Simply providing a reported result, with nothing else, is often misleading. A reported result is only complete when accompanied by a “statement of its uncertainty.”

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 us assume that a blood test result was .100. Let us also assume, based on a review of the machine’s prior performance, a “range of uncertainty” was determined to be ±5 percent, with a “confidence interval” of 99.9999 percent. This means that the reported result could be as low as a .095 and as high as a .105. This also means, if the same blood sample were repeatedly tested on this equipment, the result would only be outside of the ±5 percent range approximately one out of a million times.

On the other hand, what if for the same reported result of .100 the range was ± 30 percent, with a confidence interval of 50 percent?  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, it would fall outside the range dramatically more than the first example.  In both examples, the reported result was the same, but the second seems a lot less trustworthy (especially without knowing what caused the results outside the specifications.)

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.

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