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The purpose of the PBT is to confirm the officer's suspicion and to establish probable cause
for your arrest. The PBT is not an evidentiary alternative to the formal breath test given at the station.
The PBT is a voluntary test and the results are generally not admissible in court; most PBT devices are not sufficiently reliable to meet court admissibility standards. For example, if you had recently consumed gum, cough syrup, mints or used an inhaler prior to taking the PBT, a false positive reading could have given the officer probable cause to arrest you.
Most states prohibit the use of portable breath testing results as evidence at trial
in a DUI case.
The challenge typically goes back to the case of Duabert v Merrell Dow Pharmaceuticals (509 U.S. 579 (1993))
n Daubert, the Supreme Court held that federal trial judges are the “gatekeepers” of scientific evidence. Under the Daubert standard, therefore, trial judges must evaluate proffered expert witnesses to determine whether their testimony is both “relevant” and “reliable”, a two-pronged test of admissibility.
The relevancy prong: The relevancy of a testimony refers to whether or not the expert’s evidence “fits” the facts of the case. For example, you may invite an astronomer to tell the jury if it had been a full moon on the night of a crime. However, the astronomer would not be allowed to testify if the fact that the moon was full was not relevant to the issue at hand in the trial.
The reliability prong: The Supreme Court explained that in order for expert testimony to be considered reliable, the expert must have derived his or her conclusions from the scientific method.
The Court offered "general observations" of whether proffered evidence was based on the scientific method, although the list was not intended to be used as an exacting checklist:
Empirical testing: the theory or technique must be falsifiable, refutable, and testable.
Subjected to peer review and publication.
Known or potential error rate and the existence and maintenance of standards concerning its operation.
Whether the theory and technique is generally accepted by a relevant scientific community.
Although trial judges have always had the authority to exclude inappropriate testimony, previous to Daubert, trial courts often preferred to let juries hear evidence proffered by both sides. Once certain evidence has been excluded by a Daubert motion because it fails to meet the relevancy and reliability standard, it will likely be challenged when introduced again in another trial. Even though a Daubert motion is not binding to other courts of law, if something was found not trustworthy, other judges may choose to follow that precedent. Of course, a decision by the Court of Appeals that a piece of evidence is inadmissible under Daubert would be binding on district courts within that court's geographic jurisdiction.
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