Helping Decide Guilt or Innocence

by jeeg 20. December 2012 21:27


A man will go on trial for attempted murder in Brooklyn. His fate may hang on the handlebars of a bicycle.

Prosecutors contend he shot a man from the bike, and may have left a DNA calling card on its surface. He may not have been the only one to touch the bicycle, though.

As DNA evidence is increasingly used in courtrooms, forensic scientists are struggling to find more-precise ways to analyze smaller and smaller samples that contain multiple contributors — scraped from, say, the trigger of a gun, the handle of a knife or the trim of a scarf.

Two scientists at the forensic biology laboratory of the New York City medical examiner’s office have created a way. The scientists, Theresa A. Caragine and Adele A. Mitchell, produced the Forensic Statistical Tool, an algorithm for a software program that can analyze a mixture of DNA from a crime scene and determine the probability that it could include the defendant’s profile.

“We said, ‘Here’s a problem. Let’s do something about this,’ ” Dr. Caragine, the deputy director of the Office of the Chief Medical Examiner’s forensic biology department, said in her office last week. “I like to think of it as the New York way. Just do it.”

The tool has been used in New York City courts since July 2011, when it was approved by the New York State Commission on Forensic Science, which also accredits public laboratories, and the DNA Subcommittee, made up of statisticians and geneticists who review protocols.

What makes it innovative, but also controversial, is that in cases like the bicyclist’s, where previously no conclusion could be made, the Forensic Statistical Tool, or FST, can now use the data to produce a ratio of the likelihood that the defendant’s DNA is in the mixture compared to the DNA of random people. A lower value may help establish that a suspect is not the source of the DNA, while a higher value could point to a suspect.

In the bicyclist’s case, the FST found that there was a relatively low likelihood that the suspect’s DNA was part of the mixture on the left handlebar, but that the probability of it being part of the mixture on the right handlebar was quite high — 972,000 times more likely than the DNA of three random people.

The two scientists said that in general, differences might arise because of the amount and quality of DNA collected.

The Legal Aid Society filed a motion for a Frye hearing, which challenges the general scientific acceptance of new technology, to stop the introduction of the FST’s conclusions as admissible evidence.

There are Frye hearings on FST scheduled in Manhattan, Brooklyn and the Bronx.

The first hearing, on both FST and DNA collection, began on Wednesday in Brooklyn. Dr. Caragine presented four hours of scientific testimony while the defendant peered numbly at the PowerPoint slides.

“It’s very important, because DNA has such a powerful reputation in the legal community and the public, where it enjoys a gold standard. But it’s not that simple,” said Jessica Goldthwaite, one of the lead Brooklyn Legal Aid lawyers. “The FST must be subjected to scrutiny outside their inventors’.”

The scientists say they are confident. “If I was nervous that it wasn’t going to prevail, then we wouldn’t be using it today,” Dr. Caragine said.

Both 43 years old and mothers of boys under 2, Dr. Caragine and Dr. Mitchell work on the 13th floor in the $290 million laboratory that opened in Kips Bay, in Manhattan, in 2007. As the largest government forensics laboratory in the country, it examined 43,000 items for DNA last year. The department seems striking for another reason; it is 70 percent women, but that is actually common in forensic biology.

Dr. Caragine said the environment was supportive, rather than competitive. Her office has a plaque about teamwork, and there’s also a signed book from the crime writer Patricia Cornwell and, one day last week, there was a cold cappuccino. Dr. Mitchell’s office contains her foldup bike for her commute to Park Slope, Brooklyn.

Dr. Caragine and Dr. Mitchell are a well-balanced duo, finishing each other’s sentences the way they finish formulas. Neither has ever watched “C.S.I.”

Dr. Caragine, who invented a DNA swab out of yarn in 2003 because she found the cotton ones insufficient to gather evidence, boldly proposes new ideas. Dr. Mitchell, a trained mediator, mathematician and private investigator (with her family’s business in Seattle), tells her quietly when ideas do not work.

After earning her doctorate in molecular oncology and immunology from New York University, Dr. Caragine, from Mount Vernon, N.Y., started working for the medical examiner’s office in 2001. She identified victims from the 9/11 attacks through DNA. “It was a place where you felt useful,” she said.

In 2008, she hired Dr. Mitchell, who holds a doctorate in human genetics and molecular biology from Johns Hopkins University, to design the FST.

With the assistance of other deputies, researchers and programmers, Dr. Caragine and Dr. Mitchell spent two years validating the tool, conducting 557,000 test runs.

“We’d be partway through something and then find a better way to do it,” Dr. Mitchell said. Dr. Caragine added, “There was never a moment that we didn’t think it was going to work.”

Mechthild Prinz, the director of the lab, praised their partnership. “Theresa really has the vision and the creative force, and Adele has the mathematical skills and the understanding of the problem,” Dr. Prinz said, crediting Dr. Caragine’s “chutzpah.”

For their city service, Dr. Caragine and Dr. Mitchell won the Fred Hayes Award in 2012, named for Mayor John V. Lindsay’s budget director whose hallmark was recruiting undaunted young talent. They split the $15,000 for traveling to international conferences.

In a tour of the lab last week, the researchers demonstrated the FST. From evidence collected by the New York Police Department, the lab uses high-sensitivity DNA testing to gather small amounts drawn from skin cells off objects possibly touched by the suspect. (Blood and semen usually produce larger DNA samples, often from one source.)

The lab interprets that DNA and puts the results into the FST, which compares it to the DNA profile drawn from the suspect. The tool factors in if a piece of DNA is missing or an extra one is present, which can happen with very small or degraded samples.

It then estimates the probability that the suspect is a contributor to the mixture of DNA versus a random member of the population, and how frequently one would expect to see the pieces of DNA from the evidence in the population. The FST estimates this based on four general New York City ethnic groups and reports the lowest likelihood ratio, underestimating the weight of the results, the scientists say, in favor of the defendant.

For now, the tool can only interpret a mix of up to three DNA samples and because of the similarities of DNA within families, it cannot be used to analyze samples from people who are related.

One former member of the DNA Subcommittee said he was concerned the formula was based on assumptions, not practice.

“As a pioneer, I am an advocate of DNA work,” said Ranajit Chakraborty, the director of the Center for Computational Genomics at the University of North Texas. “At the same time, we should not push the envelope beyond what it is.”

He eventually voted for it, in part because he said the technical discussions became “tedious.”

Dr. Caragine said that from July 2011 to May 2012, they did 511 calculations with the FST, and 34 percent of the time, she noted, the evidence tended to exonerate the defendant.

“It is what it is, the evidence,” Dr. Caragine said. “Sometimes it may free an innocent person or it may solve the crime. You don’t always think about how it has a personal effect on people. It’s easier not to think about it — we just have case files here and it’s very clinical.”

Liz Robbins, NY Times



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