Retrograde Extrapolation – Starting to Be Recognized as Junk Science
Forensic science, the science law enforcement uses to try and help solve crimes and get convictions, is generally viewed by actual research scientists as the least reliable field of science. Whether it is ballistics, finger prints or the analysis and application of blood alcohol testing, there are few scientific standards, hardly any peer reviewed articles or reliable studies and a sharp divide in opinion between what police department paid experts opine and what experts in private practice will testify to.
Recently, one of the more unreliable methods used in drunk driving cases has been coming under more and more scrutiny. Its fancy scientifically reassuring sounding name is retrograde extrapolation. It is used in DUI cases by prosecutors and their “experts” to try and get convictions in both alcohol and drug DUIs. But, when you understand the scientific realities, it is easy to see how unscientific retrograde extrapolation is.
Here is how it is used in a run of the mill DUI case. The police almost always ask these two questions in a DUI investigation. How much did you have to drink? And When did you finish drinking? Now, understand, almost everyone answers, “A couple beers.” And “I finished hours ago.” They may be fudging the truth, but, it is almost a knee jerk reaction by a person who does not want to be arrested for DUI. If you do not know how DUI prosecutions work, you think these are the best answers to give to convince the cop not to arrest you. In fact, in the long run, they are the worst possible answers. The cop really does not believe anybody when they give those answers, and, the prosecutor is going to use them to try and convict you.
Here’s how it plays out in court.
- The prosecutor calls an “expert” from the crime lab. This person may have a chemistry degree. What you can be sure of is that they have been through ”training” at the lab. This “training” was summed up by one San Diego Police Department Crime Lab “expert” as having two parts. First, they are given a stack of studies and articles and told to read them because that is what they are going to use to form their opinions. That stack is comprised of the studies that say breath and blood testing is perfect and the other conclusions needed so the “expert” will testify to juries in just about every single DUI trial, the Defendant must be guilty. They are not given the many, many articles and studies that question or contradict the conclusions of the studies that are good for the prosecution. Sadly, this isn’t education, it is indoctrination. The second part of their “training” is correlation studies where they go have people drink alcohol and do field sobriety tests in a nice well lit room, not the side of the road, with no stress involved since they are not going to be arrested.
- The expert will testify that it could not have been a couple beers given the test results. Now, the prosecutor can call you a liar.
- Then comes the retrograde extrapolation. They assume you told the truth about when you finished drinking. So, all the alcohol must have been in your system when you were driving. Retrograde extrapolation is simply adding 0.01% to 0.02% for each hour between when you were stopped and when you did your blood or breath test do to your body burning off alcohol. So, the prosecutor will tell the jury you were even drunker than your test says.
So, why is this scientific garbage? First of all, retrograde extrapolation is only possible if you know when the person stopped drinking. You give them two answers. One, the number of drinks, they say must be a lie. The second, the time you stopped, they assume is true. What real scientist relies on information from a source they believe lied in one of two answers? No real scientist would. Forensic scientists do so they can accomplish their goal, trying to help the prosecutor convict people of DUIs whether what they say it accurate or not. Furthermore, the numbers they use for the rate of burn off are vague averages and do not necessarily apply to any particular person at any particular time. Lastly, they assume, again based on the few studies they are told to rely on, very set times for absorption of alcohol, that it is very rapid and happens very predictably, when in fact, medical studies show that is not necessarily true. Again, the forensic “expert” is dumbing down the actual science, not to make it simple, but, to make it as pro-prosecution as possible.
A very good Judge I know with years and years of experience once confided in me. He said it used to be that years ago, you had to watch Defense experts closely or they might mislead the jury. Now, the worst experts misleading juries are coming from the crime labs.
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