by Dale Chappell

Scientific experts have long relied on objectivity to reach conclusions—the ability to prove repeatedly the outcome of an analysis no matter who’s doing the analysis. Forensics experts, on the other hand, have long relied on subjective analysis to reach conclusions, basing the outcome on personal experience and opinion.

Over the last decade, at least, science experts have urged forensic experts to become more objective in their analyses of crime-scene evidence.

In 2009, the National Academy of Sciences (“NAS”) released the report, Strengthening Forensic Science in the United States: A Path Forward. The goal was to make forensic investigations "as objective as possible so the results do not depend on the investigator.”

The report identified the problem with subjectivity in forensic tests, and highlighted fingerprint analysis, where the opinion of two forensic examiners could differ over the same fingerprint. However, courts have upheld the subjective analysis of fingerprint identification as “acceptable and reliable.”

In United States v. Herrera, 704 F.3d 480 (7th Cir. 2013), U.S. Court of Appeals Judge Richard Posner likened analyzing fingerprints to the opinions of art experts: “Matching latent fingerprints is thus a bit like an opinion offered by an art expert asked whether an unsigned painting was painted by the known painter of another painting; he makes or rejects a match on the basis of visual evidence.” Posner concluded that “ultimately the matching depends on subjective judgments by the examiner.”

DNA evidence used to be analyzed the same way—30 years ago. But technology has made DNA evidence perhaps the most reliable physical evidence linking (or not linking) an accused to a crime scene. Today, DNA alone can exonerate a convicted person on death row with little debate.

So, how did DNA evidence become so reliable? In 1989, a trial in New York involving DNA evidence shook forensic experts, changing their thinking. The prosecution scientist and the defense scientist got together and examined the evidence. They found that there were no standards or objective criteria for assessing DNA evidence from a common source.

One of the scientists, Dr. Eric Lander, president of the Board Institute of MIT and Harvard, later said “testing labs were giving, blithely, numbers like one in 10 billion for the chance of a match, when, in fact, when you probed underneath it, there was no real evidence to support those claims.” He blamed the lack of objective standards in DNA analysis.

NAS issued reports in 1992 and 1996 suggesting some objective techniques for analyzing DNA.

While law enforcement argued that science would undermine the reliability of DNA evidence, the exact opposite happened. “In the end,” Lander said,” it became a “tool for truth.” To date, this remains the most well-studied and convincing form of forensic evidence.

Karen Kafadar, a professor of statistics at the University of Virginia and co-author of the 2009 NAS article, argues in Significance magazine (April 2019) that “DNA analysis remains the most well-studied and convincing form of forensic evidence.” She points out that an objective approach provides three benefits: (1) consistency, (2) validation, and (3) certainty in the conclusions.

As for consistency, she noted that “an important goal in any scientific process is to reduce, as much as possible, any potential grounds for criticism of the results.” The same should hold true for forensic analysis, she said. Without consistency, there may be “justifiable grounds for criticism,” she warned.

The scientific community agrees the more objective the approach, the more agreement there will be among analysts. Kafadar gave an example of forensic analysis using different methods to examine a single fingerprint. Because the subjective approach is accepted for fingerprint analysis, “the examiners may well come to different conclusions about the strength of the evidence,” she said.

In 2012, a group of experts created a flow chart for the fingerprint examination process. By focusing on exact steps, examiners were able to consistently reach a valid conclusion, and this increased certainty in the outcome.

“The main goal should be in defining the process so that the result has as little uncertainty as possible,” Kafadar said. Analysis with “quantifiable, objective steps is easier to validate, because all can agree on the points of the process,” she said.

Not all evidence must be like DNA to be reliable, Kafadar said. But the protocols used to analyze DNA are scientifically justified as a series of steps that lead to consistent results. A more objective approach to forensic analysis would provide greater confidence in the reliability of crime scene evidence. “Ultimately, this raises everyone’s confidence in the criminal justice system,” Kafadar said.

Source: significancemagazine.com (Royal Statistical Society)

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