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Is retrograde extrapolation junk science?


Judges are supposed to be rigorous in determining the validity of evidence allowed in their courtrooms. Especially when it comes to scientific evidence, which most laypersons on a jury and the majority of judges are ill equipped to evaluate.


There are various standards, but ideally, the process should be accepted science, that is well understood and can be used to provide consistent results. Sadly, there are numerous incidents of poor science being used in courtrooms to obtain convictions. In what may be the most egregious example, one man was executed based on now generally discredited "junk science" that was used in an arson investigation. 


Another type of evidence that has been questioned, known as retrograde extrapolation has been used in many states as a method of guessing a person's blood alcohol content (BAC) levels at the time of an arrest or accident, when law enforcement only has available a measurement that was taken hours after the incident.

While it has some basis in science, there are a large number of variables in play when making the calculation. What the person was drinking, how much they drank, during what period, what they ate, their weight and the rate their body metabolizes alcohol, all of which can compromise the accuracy of any "extrapolation" that is based on averages and assumptions.

One DUI attorney notes that it is about as reliable as astrology, but prosecutors use it because many courts have permitted its use in the past and that habit provides some element of credibility that it otherwise might not have.

If you have been charged with a DUI in Minnesota, you should always challenge any testimony arguing the application of retrograde extrapolation. Because of the complexity of this determination, it may be possible to show the jury that such a determination is questionable, and not beyond a reasonable doubt.

Source: ap.org, "How drunk? Estimates questioned in drunken-driving cases," FRANK ELTMAN, October 4, 2015

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