How a DUI Attorney Can Use Logic to Show that Field Sobriety Tests are Unreliable

Logic is sometimes defined as the study of principles governing correct reasoning. Logicians like to distinguish between the principles of correct reasoning and the psychology of correct reasoning. However the ideas are similar in that they both include a measurement of principles governing the validity of arguments. In other words, logic is a determination of whether certain conclusions follow from given assumptions. The problem with logic presents itself when the given assumptions are defective. Apply this problem to field sobriety tests.

Premise: If you are drunk, then you are uncoordinated.

This is the starting point for the invalidity of field sobriety tests, because it is a faulty premise. In other words, it is an assumption that is not accurate. Let’s use a more outlandish example to better illustrate my point for this article. If you are good at football, then you are fast. Similar to the drunk premise, at first glance it seems correct. After all, there are some fast guys in the NFL. But there are also some slow guys – offensive lineman or kickers, for example. So the premise is faulty.

Inverse: If you are uncoordinated, then you are drunk.

This is the hallmark of the field sobriety tests. The tests assume that failure equals inability to safely drive a vehicle because the test taker is too uncoordinated to drive. Even if you assume that the tests can accurately measure coordination, coordination is not a measurement of impairment. Going back to my second premise, the inverse would be that if you are fast, then you are good at football. Most of us would agree that this isn’t true. I like to think that I was fast, at least during my high school and college days, but I sucked at football.

Converse: If you are not drunk, then you are not uncoordinated.

Put another way, the converse states that if you are not drunk, then you are coordinated. Trust me on this one, the converse is absolutely false. Remember the kid in elementary school who was always picked last for kickball? That kid was not coordinated. But that kid also wasn’t drunk. The converse of my second example is that if you are not good at football, then you are not fast. Look at Usain Bolt, the world champion sprinter. He is not good at football, yet he is the fastest man alive.

Contrapositive: If you are not uncoordinated, then you are not drunk.

In other words, if you are coordinated, then you are not drunk. Also false. This might seem counter-intuitive, but people successfully drive their vehicles after drinking too much every day. So the statement that coordination equals not drunk is erroneous. Likewise, to say that if you are not fast then you are not good at football is also inaccurate. Again, I point to the NFL offensive lineman – generally not considered fast, but still good at football.

In effect, we are applying unreliable tests to determine that people are guilty. So why does all of this matter? Our criminal system is founded upon the principle that it is better to let 99 guilty people free then to convict 1 innocent person. That is why we have the “beyond a reasonable doubt” standard. So why do we rely on inherently unreliable tests to judge DUI defendants?

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