Assuming that was his real purpose

The other day I was in county court handling a client's DWI case. The judge was calling the trial docket and trying to figure out if anyone was going to trial that day. One by one came the requests for a continuance - and one by one they were granted. That is, until there were two cases left.

One of the cases, a breath test case, was the one picked to go. The defense attorney asked the judge for a continuance because he wanted to talk to the technical supervisor about the assumptions he made in the process of guessing extrapolating the defendant's alcohol concentration at the time of driving.

I overheard bits and pieces of the conversation at the bench while I waited (and waited and waited). The defense attorney wanted to know what assumptions the technical supervisor was making. The judge insisted that the technical supervisor was assuming only one thing - that the defendant had nothing to drink between the time of his arrest and the time he blew into the little black box. The judge insisted that technical supervisors aren't assuming anything else. She told the defense attorney that they use the time of the last drink and of the last meal and the breath test score and the time that elapsed between the time of driving and the time of the test. The defense attorney said the technical supervisor was also assuming that his client was in the elimination phase at the time of the test.

I couldn't help but think that while the defense attorney was correct in his statement that the technical supervisor was making assumptions, it was a pretty lousy argument to use if you wanted to get the case continued.

Of course the technical supervisor is making assumptions. The judge was so far off base with her assertion that he wasn't. Let's take a look at some of the facts used by the technical supervisor to guess extrapolate a driver's alcohol concentration.

First the technical supervisor needs to know about how much the defendant weighed. He also needs to know how much time elapsed between the time of driving and the breath test. He needs to know when the defendant consumed his first and last drinks and when he ate his last meal. It would be helpful if he knew exactly how much the defendant had to drink.

Now let's think of the assumptions being made by the technical supervisor. First he is assuming that the breath test result is accurate. He is assuming that the machine was operating properly that evening and that the operator conducted the test properly. He is assuming that the operator observed the defendant for at least 15 minutes prior to the test. He is assuming that the defendant is being truthful when asked about the time of his first drink, last drink and last meal. He is assuming that the defendant isn't being truthful when he told the officer he had only had a couple of beers. He is assuming that the defendant is in the elimination stage. He is assuming that the defendant eliminates alcohol at a steady rate equal to the number he plugs into his calculator. Finally he is assuming that armed with one data point it is possible to calculate the slope to a another data point at some time in the past.

So, yes, there are a lot of assumptions being made by the technical supervisor. And each of those assumptions gives rise to a line of questions during cross-examination. The best way to attack the conclusions of an expert is not to challenge the expert head-on. The best way is to challenge the assumptions he made along the way.