Let’s Talk Numbers

I am a certified latent print analyst and crime scene investigator for a local police department in Florida. Before that I worked at a coroner’s office in Cleveland Ohio.

Defense attorneys have grown fond of pointing out that fingerprint examiners cannot prove that there are not two people in the world with the same fingerprint. This is true, obviously, because not everyone in the world has been fingerprinted and even if they were, some are dying and new ones are being born every second. But I also can’t prove that unicorns don’t exist, as simply because no one has ever seen one doesn’t prove that they’re not there. They might just be really good at hiding. As every lawyer learns on their first day of law school, you can’t prove a negative.

DNA has spoiled us. With their charts and their tables and their mathematical calculations of the ‘the frequency of occurrence of this profile in related individuals is one in 48 quadrillion’. Humans love reducing things to numbers, quite understandably—numbers are easy to write, easy to read, easy to understand, objective and uniform. And when you’re in a court of law and someone is looking at the death penalty, objective and uniform are what you want. But make no mistake about it: ‘frequency’ is just another term for ‘odds.’

 And thus fingerprint science is being pressed for a similar system of numbers and odds. The problem is that fingerprint results are either yes or no—yes, they match, or no, they don’t. Normally this binary system is the ideal in a court, but it doesn’t allow for all the tables and calculations. You can’t divide, multiply or take a percentage of zero, as we all learned in basic math. A zero category doesn’t leave you much to work with.

But zero assumes that there are not two people with the same fingerprint, so let’s look at some other numbers. One person’s set of fingerprints consists of ten fingers, and two palms which are each divided into ten sections roughly the same size as a fingerprint. So if I compare one latent print collected at a crime scene to one person, that is thirty comparisons. At my department we receive or collect, at a very conservative estimate, two latent prints per day. Our database of known fingerprints is currently over 56,000. 2 x 56,000 x 30 = 3,360,000. Over three million comparisons per day. Yes, some of these records are duplicates (or as we call them, frequent fliers) so let’s round down to an even three million per day, 365 days per year, at one police department in one small and relatively low-crime city. There are approximately 20,000 police agencies in the US.

This is not even considering what the FBI is doing with their repository of 50 million sets of fingerprints.

I don’t think my calculator has enough spaces to work that out.

But just in case we ever do find two people with the same fingerprint and I’m out a job, I’ve got another one lined up. I write novels, in which the heroine is (natch) a forensic specialist. My fifth is called Trail of Blood and comes from a true story: During the Great Depression, the city of Cleveland was terrorized by more than the world’s turmoil: a madman, who murdered by night and left his handiwork to be discovered in the first glare of day. He was America’s version of Jack the Ripper, but more prolific. He was bloody. He was unknown. He was never caught. These are facts.

Trail of Blood begins where history leaves off, and forensic scientist Theresa MacLean must use history and science to discover the secret behind these frenzied crimes of both past and present.

So buy a copy. Just so I can hedge my odds.

Lisa Black spent the five happiest years of her life in a morgue, working as a forensic scientist in the trace evidence lab until her husband dragged her to southwest Florida. Now she toils as a certified latent print analyst and CSI at the local police department by day and writes forensic suspense by night. Her books have also been published in the United Kingdom, the Netherlands, Germany, France, Spain and Japan. For more information visit her website: www.lisa-black.com.

Author: Lisa Black

I am a forensic scientist and author of 5 novels of suspense.

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