YourABA: June 2013
YourABA June 2013 Masthead

Ensure that mathematical proof adds up

Although lawyers like to use statistics to their advantage, problems can arise if numbers are employed without careful thought, said Stephan Landsman, a professor at DePaul University College of Law in Chicago. In an ABA Sound Advice podcast from the Section of Litigation, Landsman reviewed People v. Collins for a “host of important lessons about relevance, trial practice and mathematical proof.”

The case, decided by the California Supreme Court in 1968, began with the mugging of Juanita Brooks in an alley of the San Pedro neighborhood of Los Angeles. “Mrs. Brooks’ purse was snatched from behind, and all she could say was that her assailant was a blond woman with a ponytail who jumped into a yellow car driven by a black man with a beard and a mustache,” Landsman said. “There was an eyewitness to the mugging, but she could provide no more positive identification than could Mrs. Brooks.”

The Los Angeles police department arrested Janet and Malcolm Collins for the crime. “Janet had blond hair that she wore in a ponytail; Malcolm was a black man who at the time of his arrest had a mustache but no beard,” Landsman said. “The couple owned a yellow Lincoln with an off-white top.”

At trial, Landsman said, the state offered the following evidence: testimony that when the police came to arrest the Collinses, Malcolm fled out the backdoor of his house; testimony that Malcolm had a beard the day after the purse snatching; testimony that the victim had $40 in her purse and that Malcolm paid two traffic fines totaling $35 the day after the robbery, though neither he nor Janet were gainfully employed; and finally the testimony of a mathematics professor who described the product rule used to calculate by multiplication the odds of an event occurring as an outcome of a sequence of independent factors leading to a certain result. “The prosecutor then asked the mathematics professor to calculate the likelihood of there being more than one couple having the following characteristics for which the prosecutor provided the odds,” Landsman said.

“A partly yellow automobile: The prosecutor suggested odds of one in 10,” he continued. “A man with a mustache: The prosecutor suggested odds of one in four. A black man with a beard: The prosecutor suggested odds of one in 10. A woman with a ponytail: Again the prosecutor suggested one in 10. A woman with blond hair: The prosecutor this time said one in three. And finally, an interracial couple in the car: The prosecutor here suggested odds of one in 1,000.”

The jury convicted the couple, and the case wound up in the California Supreme Court.

When a lawyer seeks through expert testimony to attach numbers, the court and opposing counsel need to take great care.

Landsman offered some questions to consider about the matter. First, was Malcolm’s flight from the police relevant in the prosecution? “The answer is yes,” he said. “Although it might not be motivated by fear related to this crime, it does suggest a guilty state of mind, and under Federal Rule 401, evidence having any tendency to increase the likelihood of a fact in issue is admissible. Virtually no court in the United States would exclude this evidence.”

A second question: Are the beard and traffic fine payment evidence relevant in this particular proceeding? “Yes, they clearly have a tendency to increase the likelihood of guilt and carry no sting of prejudice at all,” Landsman said.

“Might there be other explanations for these factors? Of course there might, but they’re still relevant,” he continued. “Is there enough to convict the Collinses without the mathematics testimony? I don’t think there is. The case is simply speculation. There is no proof that the Collinses were nearby when the crime was committed or that they were involved. The key to the case then must be the mathematical evidence.”

Is there anything wrong with the way the mathematical proof was presented to the jury? “Yes,” Landsman said. “The prosecutor supplied the odds. In other words, he became a witness and was testifying at the proceedings. As you all undoubtedly know, that is improper. Moreover, are these odds based on the real world? How do we know as a mathematical fact that one man in four might have a mustache? I don’t think we can know that.

“How do we know that on the streets of Los Angeles, one in 1,000 couples in a car is an interracial couple, let alone an interracial couple with a black man with a beard and a mustache driving and a white woman as a passenger? This material is absolute garbage improperly proferred at trial and used to convict in an otherwise weak case.”

The mathematics itself is “garbage” as well, Landsman said. “The variables must all be independent if they’re going to be multiplied together,” he said. “Is it more likely that a man with a mustache will also have a beard? If so, giving odds twice for both of those facts is in essence double counting. And of course the same analysis might be applied to most of the other factors that the prosecutor provided.”

What are the lessons of the Collins case? “While circumstantial evidence is fine, one must be careful with it and in evaluating its total worth,” Landsman said. “When a lawyer seeks through expert testimony to attach numbers, the court and opposing counsel need to take the greatest of care. They need to ask, is the underlying math truly reliable? Is it truly applicable to the case that’s being tried? Does it pose a danger of misleading, confusing or distracting the jury? If so, it should be barred. There is good mathematical evidence, especially related to things like DNA, but often, as in Collins, mathematics is improperly used.”

Sound Advice is a series of podcasts provided by the Section of Litigation.

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