Yes, the BCA Really Does Hide Bias in Breath Testing

Posted On February 20, 2023 Charles Ramsay

A few weeks ago, I wrote about how the BCA breath alcohol lab hides bias in its measurements. Instead of adjusting for systematic bias, they treat it as if it were a normal distribution in their uncertainty equations.

I received several comments on LinkedIn expressing utter disbelief that the BCA would treat bias in such a manner. But the fact of the matter is, it’s true! 

What is bias, AKA systematic error?

If you’re not up on the science, here’s what you need to know about bias -- it is the systematic overestimation or underestimation of a measurement. Let’s take an example: If you step on a scale and it consistently overestimates your weight, the scale has a positive bias. 

The figure below shows an illustration of bias. You can see how bias is the difference between the measured value and the true value.


What is random error?

Another type of error is called random error. All measurements, no matter how precise, have some random error. For example, if your bathroom scale sometimes weighs 1 pound heavy, and other times 1 pound low, but usually right around your weight, that is called random error.  

Don’t confuse random and systematic error

The problem comes in when the BCA confuses the two types of errors.

The BCA treats bias in their breath alcohol machines as if it were just another source of random error. This is a major problem because, in reality, bias is either negative or positive. It isn’t normally distributed like a typical bell curve.

If you don’t believe that the BCA commits such a foul, you can read the BCA’s uncertainty procedure for yourself. Here are a few quotes from their procedure to explain it to you in plain English:

“Measurement bias is accounted for in the combined standard uncertainty.”


“The bias is assumed to have a uniform distribution. “

For those who can understand math, check out their formula below:

Why is this a problem?

I’ll say it again and again -- this is a problem not only because it is bad science, but because it gives a false picture of the true breath alcohol measurement.

The lab does not determine the systematic error and correct the measurement as the authoritative guide to uncertainty, the GUM suggests. 

Instead, the breath alcohol scientists tell judges and juries that the “most likely result” is the mean of the two measurements taken. In cases where the burden of proof is “preponderance”, i.e., greater than 50%, this misrepresents the true measurement. 

For example: In a breath alcohol test case with two samples of 0.082 and 0.084, where 0.08 is the legal limit, the lab’s scientists will say the “most likely result” is 0.083. They'll testify that the uncertainty is approximately +/- 0.01, but they won’t tell the judge about how they should’ve adjusted for bias first. 

Time for a change

If the BCA simply followed the science, their testimony would be much easier. They wouldn't have to squirm in their seats when confronted with scientific documents in court.

For years I told them they needed to implement uncertainty. They finally did, but they chose to do it improperly. 

Hopefully, it won’t take as many years for them to fix their uncertainty as it did to implement it.

If you’ve been a victim of the State's careless science, reach out to Ramsay Law. We take time to understand the science and explain how the State doesn't follow it.