July 01, 2015

Uncertain Justice Measurement: Uncertainty and the Discovery of Truth in the Courtroom

By Ted Vosk

Editor's Note: A full version of this article that includes endnotes and charts is available from the author, who can be reached at tvosk@comcast.net.

Forensic measurements play a prominent role in today’s justice system. They are relied upon to investigate and prove criminal and civil charges ranging from murder to simple traffic offenses. Even establishment of some crimes and punishments requires forensic measurements. For example, the crime of operating a motor vehicle with a breath alcohol concentration (BrAC) in excess of a particular limit and the enhanced sentences that may result cannot be established absent forensic measurements.

Yet as ubiquitous as forensic measurements are, they are often poorly understood and misrepresented by forensic and legal professionals alike. This undermines not only the ability of fact finders to properly weigh such evidence but the public’s confidence in verdicts based upon it. If forensic measurements are to fulfill their promise as a tool for the determination of truth in the courtroom, they must adhere to the requirements of science. Failure to do so divests them of the power science gives them.

A fundamental aspect of science is that no result permits the truth of a proposition under investigation to be known. The best scientific inquiry can do is assess whether a proposition is more or less likely. Only by understanding the limitations of science can those deciding facts understand the constraints upon their conclusions.

Forensic measurements are no different. They cannot reveal a quantity’s true value. For measured results to facilitate the discovery of truth in a courtroom, forensic scientists must communicate the limitations of the conclusions they support—the results of all forensic measurements must be accompanied by their uncertainty when admitted in evidence.

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