Examination of a Prosecution Expert

Examination of a Prosecution ExpertDuring a DUI / DWI trial, the second witness that the prosecution will call is usually a chemist from the crime lab – their “expert”. This testimony always points toward a defendant’s guilt, but a skilled drunk driving criminal defense lawyer will not only cross-examine the expert, but also call defense experts who will contradict the prosecution witness’s testimony.

The prosecution will start by having the chemist rattle off all of his or her education and training in an effort to build credibility.

These experts almost always testify as to presumptions regarding the defendant’s level of intoxication based on general scientific principles and assumptions. These assumptions do not take into account a defendant’s tolerance level, size, absorption rate, or meals eaten prior to drinking. The expert will testify about any chemical tests the driver took, including how they were performed, the accuracy of the tests, and what this says about the level of alcohol in the defendant’s system.

The expert may then discuss how field sobriety tests assist an officer in determining whether or not a driver is under the influence. The expert may then offer an opinion as to how well the defendant performed on the tests and what this says about the driver’s blood alcohol content (BAC).

The prosecutor may ask the expert testify about how alcohol affects a person both mentally and physically, and specific examples of behavior that demonstrate a person is under the influence.

Finally, the prosecution’s expert may make hypothetical statements about “hypothetical situations or drivers” and use these situations and conclusions to support the charge that the defendant in this specific DUI trial was also under the influence. Remember that experts can rely on hearsay evidence (out of court statements offered to prove the truth of the matter asserted) in making their opinions, hypotheticals, and conclusions.

Because the prosecution’s expert is able to make hypothetical assumptions about a driver based on hearsay, it is vital to the defense of a DUI / DWI charge that the person accused retain a qualified and experienced DUI criminal defense attorney.

After the prosecutor’s direct examination of its expert, the defense attorney begins to cross-examine the witness. An experienced DUI lawyer can use this opportunity to discredit the expert and dismantle the prosecutor’s case. Just as in the cross-examination of the arresting officer, grilling the prosecution’s expert in a DUI / DWI case is less of an examination than a chance for the defense attorney to testify. The attorney’s leading questions are designed to elicit only a yes or no response. This allows the defense attorney to testify to what he or she wants the jury to hear, and then forcing the prosecution’s witness to agree with that testimony.

An effective DUI / DWI defense attorney knows it’s better to score points with the prosecution’s expert than it is to score points with a defense expert. The jury knows the defense expert is hired by the defense to help the defense. But the prosecution’s expert is in opposition to the defense, so to persuade that witness to agree with the defense, or support something that helps the defendant’s case, is much more valuable than anything the defense expert could say.

The practiced DUI criminal defense lawyer will use cross-examination to point out that the expert doesn’t know anything about the defendant’s drinking pattern, level of tolerance, or how his or her body absorbs alcohol. The expert does not know how the defendant drank the night he or she was arrested, or how much. The expert does not know if the defendant has any injuries or illnesses that would impact the performance of field sobriety tests, or could affect the sample provided during the chemical tests.

It is also important to point out on cross-examination of the expert that the witness is part of the prosecution team. Very often, the prosecution’s expert is employed by the local sheriff’s department. This can also help to discredit and weaken the impact of the prosecution’s expert.

Without cross-examination from an attorney who specializes in drunk driving defense, the prosecution expert will only testify about information that will help convict the driver. With a skilled cross-examination, the impact of a prosecution expert’s testimony can be diminished or completely eliminated.

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