Probability Statements Should Be Introduced into Scientific Testimony

A probability of a match, unless that probability is practically zero or one, would be a key first step to improving forensic science. In the case of the convicted Martin Luther King Jr. assassin, the James Earl Ray rifle was determined to be an inconclusive match to the assassination bullet by all of the several examiners used by the House Select Committee on Assassinations (HSCA). A finding of an 80%, 50%, or 20% chance of a match, along with an interval of uncertainty for that chance, would have been informative to Congress, the public, and possibly the judicial system. In current murder cases, it would provide a measure of the strength of evidence.

Current presentations in court state that when the match of the defendant’s gun to a murder bullet is inconclusive, the prosecutor can state that the bullet could well have come from the defendant’s gun. A probability for a match would be helpful to assess a proposed match. In addition, matches should be specific to a subclass of weapons. The uniqueness of firearm toolmarks has never been established. Using probability statements that can be defended is in the best interests of the defense and prosecution, and it will help the courts to make correct decisions more of the time. Such statements also are supported by the National Resource Council’s Strengthening Forensic Science in the United States: A Path Forward, which recommends establishing a National Institute of Forensic Sciences.

Methods that produce probability statements have yet to be tested and will require the cooperation of law-enforcement agencies. I was recently part of a group invited to submit a full proposal to the National Institute of Justice to test a method in District of Columbia (DC) criminal cases that would produce viable probability statements that could be defended through an application of Bayes’ theorem. While the DC Public Defender’s Service agreed to participate in the proposed research, the DC Metropolitan Police Department did not, so the full proposal did not go forward. Without the cooperation of law enforcement, having bullets and weapons available for experimentation is impossible without case-by-case litigation.

A successful test of this method would be a good precursor to trying the method on the bullet fragment recovered from Martin Luther King Jr. It would provide the probability of a match to the James Earl Ray rifle and rifles with similar subclass characteristics.

While automated identification in our proposal was not available at the time of the HSCA study, Bayes’ theorem and Bayesian methods were. Had there been input from statisticians, it is likely that information that is more useful would have been provided to Congress.

Much needs to be done to make forensic science more scientific. In “Probability Statements Should Be Introduced into Scientific Testimony,” I make the case for introducing probability statements in science testimony. That faces resistance, however, in that a key police agency has declined participation in a study of one proposed method.

The NRC report Strengthening Forensic Science in the United States: A Path Forward had many recommendations, one of which I’ll highlight. To address the identified lack of rigorous certification programs for forensic scientists, of strong standards and protocols for analyzing and reporting on evidence, and of peer-reviewed journal articles on scientific bases and reliability of many forensic methods, the report called for the establishment of a National Institute of Forensic Sciences (NIFS). Requirements for the NIFS include the following:

The NRC also noted that no federal agency exists that meets these well-considered and important criteria.

The Innocence Project, based in New York City, is taking a coordinating role in implementing many of the NRC forensic science recommendations through a coalition called Just Science. I have signed their petition to Congress to establish an agency like NIFS and would encourage readers to do the same.

It is time for forensic science to become part of the mainstream scientific community. Probability statements in scientific statements are one step. NIFS is another. Statisticians should be integrally involved by educating the public about the weaknesses of forensic science, urging and developing solutions, and working with forensic scientists.