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The Margin of Error: Why Tests Can Get It Wrong

Posted by Terry A. Wapner | Oct 16, 2013 | 0 Comments

Let's say you just got stopped for DUI, and blew a .09 into the breath machine after a few drinks.  You're arrested and brought to the police station.

The strange thing is, you're pretty sure you weren't impaired—you know you drank, but you're sure you weren't drunk.  Why does the test think your levels are over the legal limit?

It may surprise you to learn that BAC tests (including not just subjective testing like field sobriety testing but also, supposedly “objective” methods like blood and breath tests) can get it wrong.  Every BAC testing device, regardless of how it works, has a margin of error.  That margin of error may be exactly what you need to beat your DUI case.

The entire process of performing a DUI arrest is designed to make people feel like DUI testing is scientific and accurate.  If the police can get you to believe that the breath machine or the blood test is 100% accurate, 100% of the time, you won't spend the time or money trying to fight the charge.

When police arrest you with a BAC very near the legal limit, they're banking on the idea that you won't know about the margin of error.  Studies have shown that breath machine accuracy may be off by as much as 40-50%.  This means that if your BAC read .10%, it could be as low as .05%.

In any criminal case, the state is required to prove beyond a reasonable doubt that you committed the crime you are accused of.  A good defense attorney who has the scientific experience with DUI cases can help you show the doubt you need to demonstrate to get an acquittal.  The exact margin of error will depend on the exact equipment used to conduct your tests as well as other factors.  Call Fresno DUI Lawyer Terry Wapner today to get more information on fighting your DUI charge and to give you your best day in court.

About the Author

Terry A. Wapner

Terry A. Wapner confines his practice to the defense of persons accused of driving under the influence of alcohol and/or drugs, and related crimes.


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