Back to Newsroom


Forensic® News DNA Matters column on how to use the likelihood ratio

Today Forensic® News ran its second DNA Matters™ column – a Special Feature written by Cybergenetics founder and chief scientist Dr. Mark Perlin. In this general article, Dr. Perlin explains "How to Use the Likelihood Ratio."

Forensic scientists report a "likelihood ratio" (LR) for the strength of match between DNA evidence and a suspect. Without a match statistic, DNA can't be used as courtroom evidence. The entire purpose of forensic DNA science is producing a reliable LR match statistic.

But what is a "likelihood ratio"? Is it merely forensic gibberish devised to confuse judges, jurors, and lawyers? Or is it something simpler that lives at the heart of scientific information?

How is the LR used? Yes, it can connect crimes to criminals. But can it do more? Can it protect the innocent? Or prove a negative that seems impossible to show?

When is the LR easy to understand? Some likelihood ratio statements are incomprehensible to most people. But other ways of saying it are easily explained and understood.

The second DNA Matters column tells the story of a cold-blooded killer in Southwestern Pennsylvania, with DNA mixtures found on a gun and a hat. How initial crime laboratory reports gave little information. And how TrueAllele® data analysis revealed likelihood ratios that showed who did, didn't, and couldn't have committed the crime.

"How to Use the Likelihood Ratio" relates the homicide case of Pennsylvania v. Leland Davis. What happens when limited DNA analysis gives little insight, but advanced computing brings the same DNA evidence back into criminal justice? How does using better technology deliver crucial LR information that impacts society?

The DNA Matters column explores forensic identification technology – and how it affects people and society – in the context of criminal cases.

Since 2005, Forensic® has been bringing professionals the products, news and research innovations essential to advancements in both the laboratory and the crime scene. Their e-newsletters are delivered to inboxes every Monday, Wednesday and Friday.

Founded in 1994, Pittsburgh-based Cybergenetics innovates advanced computing to get more information out of biological data. Twenty years ago, founder Dr. Mark Perlin invented TrueAllele® probabilistic genotyping for automated human identification from DNA mixtures. The company helped identify victim remains in the World Trade Center disaster, and has helped exonerate ten innocent men.

Cybergenetics offers free TrueAllele screening of DNA evidence to prosecutors, defenders, police investigators, innocence groups, crime laboratories, forensic experts, and others. A free first look at critical DNA data can change the course of a criminal case.

Back to top