Three Ways to Discredit Retrograde Extrapolation in a DWI Breath Test Case
Whenever a motorist accused of driving while intoxicated agrees to submit to a breath test, the state’s star witness at trial will be the technical supervisor, the state’s expert on the breath test machine. One of the highlights of the state’s expert’s testimony is the calculation of your client’s alcohol concentration at the time of driving and the number of drinks it took to get to that point. This pseudo-scientific theory is called retrograde extrapolation.
While the prosecutor is asking the technical supervisor to determine these numbers for the jury, the technical supervisor will be sitting at the witness stand furiously punching numbers into a calculator. The testimony all seems so dramatic – until you ask the state’s expert on cross examination whether he and the prosecutor discussed his testimony before he took the stand. When asked he will be forced to admit that he calculated the numbers ahead of time.
The technical supervisor’s calculations can also be attacked through the use of the Texas Breath Alcohol Testing Program Operator Manual. Upon cross examination, the state’s expert will testify that the manual is authoritative in the field of breath alcohol testing – and this will allow you to cross examine the witness using the manual.
After the state’s expert has testified that your client’s alcohol concentration was higher at the time of driving than at the time of testing, you should ask the technical supervisor what your client had to eat or drink in the last hour before the traffic stop.
After the state’s expert admits that he cannot testify with any degree of certainty what your client consumed prior to his being stopped, ask him if he agrees with the manual’s statement that “there are three possibilities when estimating the alcohol concentration at a time prior to the test.”
When asked, the state’s expert will have to admit that the statement is true and that he cannot state with any degree of certainty whether your client’s alcohol concentration was higher, lower or the same at the time of testing as it was at the time of driving.
Another area of attack is the assumption that a person’s body eliminates alcohol at a constant rate. The state’s expert will testify that the average rate of elimination is .015 percent per hour. To calculate the driver’s alcohol concentration at the time of driving, the technical supervisor will just multiply that number times the number of hours between the traffic stop and the breath test and add it to the breath test result. Upon cross, however, he will be forced to admit that there is no scientific evidence to back up his assertion that the rate of elimination is constant. The elimination rate is affected by the amount of food in the stomach at the time of drinking, the type of food in the stomach, stress levels and physical ailments.
The key to cross examining the state’s expert is not to make a head-on assault. The technical supervisor is paid by the state to testify to the accuracy and validity of the breath test and the breath testing methodology. The wiser course of action is to attack the assumptions upon which the state’s expert bases his testimony. Then take the concessions the technical supervisor made on cross and use them to create reasonable doubt during your closing argument – when it’s too late for the state’s expert to defend his answers.
If you’ve been wrongly arrested for DWI, you need an experienced DWI attorney to defend your rights and restore your reputation. Contact Houston DWI Attorney Paul B Kennedy for more information and a free consultation.