The Friction Ridge
Written by Heidi Eldridge   

“I am 100% certain of my conclusion.” ... (But should the jury be certain?)

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FROM TIME OUT OF MIND, forensic scientists have testified to results with phrases like “one hundred percent certain,” and felt completely comfortable doing so. After all, why would we testify under oath to something that we did not believe to be true? Then, in 2009, the National Academy of Sciences report on forensic science was released, and in the aftermath, forensic scientists began to be cautioned against using this phrase and others like it. Many embraced this change, while others continue to ask: But why?

Many arguments have been made addressing the lack of wisdom in using a phrase such as “100% certain”. Here is how the most common argument goes: The assertion of one’s certainty does not equate to a scientific stance. Nothing in science is ever 100% certain. The cornerstone of scientific exploration is the formation of a conclusion, which is open to falsification.

Here’s that argument in layman’s terms: 1) I research a question. 2) I come up with the answer that I feel is the most appropriate given the data I had to examine. 3) I share my results. 4) Other people try to prove me wrong or attempt to fine-tune my answer.

Under this concept of science, the answer is never absolute. It is always subject to further testing, interpretation, and challenge. Therefore (the argument goes), if I claim that my result is 100% certain, I am tacitly admitting that my result is not scientific. For, by definition, a scientific conclusion cannot be absolute.

This argument is fine, as far as it goes, but it fails to resonate with some practitioners, particularly those who were never scientifically trained, and it fails to address the real crux of the problem: We must consider our audience.

When we testify in a court of law, our audience is not other scientists. Our audience consists of jurors. Laypeople. Watchers of CSI. The majority of these people are not scientifically trained. They expect us to bring to the courtroom that training, experience, and knowledge. And they look to us with a faith that, for some, borders on reverence. And because of this faith, we bear a huge burden of responsibility: Clarity.

Our words matter. Language is a powerful weapon. It can be used to inform, but it can also be used to persuade or mislead. We must remember that many of the phrases we use as scientists are a kind of shorthand for larger concepts that other scientists understand. But juries do not have that level of understanding. Juries accept them at face value.

When we say, “Fingerprint comparison science has a zero error rate,” we might mean that—although we know people can and do make mistakes—the application of the process, if followed correctly, will lead to the correct conclusion. But what the jury hears is, “Fingerprint conclusions are never wrong.”

When we say, “Fingerprints are unique,” we might mean there is a great deal of biological research that supports the random formation of fingerprints, and in the limited research we have done, we have not found two that were exactly the same… so we conclude that it is extremely unlikely that two fingerprints could be identical. But what the jury hears is, “It is a proven fact that every fingerprint is different.”

Similarly, when we say, “I am 100% certain of my conclusion,” we might mean that we have conducted a careful examination, reached the best conclusion possible with the data available, and that we would not have reported that conclusion unless we were confident that we had done our work well. But what does the jury hear? They hear, “I’m an expert, and I’m telling you that this conclusion is fact and cannot possibly be wrong.”

But the truth of the matter is, sometimes we are wrong. And what we are stating for the jury is not fact; it is opinion. To be clear, the opinion is based on something—it is not just made up out of thin air. But it is still opinion. And to state it in terms that give it the veneer of fact is both overstating and just plain misleading.

Remember your audience: The jury that is accepting what you say at face value. They need you to be precise in your use of language so they understand correctly. It is okay to say that you are certain—if you qualify the statement. Talk about your personal level of confidence in your conclusion. Talk about the work you did to reach your conclusion and why you feel it is correct. But do not imply that your opinion is an incontrovertible fact.

Juries do not know the subtext behind our conventional phrases. All they hear are the words we say. We need to be certain that those words truly convey our meaning. We owe it to the jurors who have placed their faith in us, the experts.

About the Author

This e-mail address is being protected from spam bots, you need JavaScript enabled to view it is a Certified Latent Print Examiner with the Eugene (OR) Police Department in Eugene, Oregon. She has successfully defended fingerprint science in a post-NAS Daubert-style Motion to Exclude hearing in Oregon and has been writing and teaching on the subject to help others understand how to meet these challenges.

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