Devlin's Angle

The Professor, the Prosecutor, and the Blonde With the Ponytail

Near the end of the alley, a man named John Bass saw a woman run out of the alley and jump into a yellow car. The car took off and passed close by him. Bass subsequently described the driver as black, with a beard and a mustache. He described the young woman as Caucasian, slightly over five feet tall, with dark blond hair in a ponytail.

Several days later, the LA Police arrested Janet Louise Collins and her husband Malcolm Ricardo Collins and charged them with the crime. Unfortunately for the prosecutor, neither Mrs. Brooks nor Mr. Bass could make a positive identification of either of the defendants. Instead, the prosecutor called as an expert witness a mathematics instructor at a nearby state college to testify on the probabilities that, he claimed, were relevant to the case.

The prosecutor asked the mathematician to consider 6 features pertaining to the two perpetrators of the robbery:

Black man with a beard Man with a mustache White woman with blonde hair Woman with a ponytail Interracial couple in a car Yellow car

According to this rule, the witness testified,

P(black man with a beard AND has a mustache) = P(black man with a beard) x P(has a mustache) = 1/10 x 1/4 = 1/(10 x 4) = 1/40

Black man with a beard: 1 out of 10 Man with mustache: 1 out of 4 White woman with blonde hair: 1 out of 3 Woman with a ponytail: 1 out of 10 Interracial couple in car: 1 out of 1000 Yellow car : 1 out of 10

Impressed by those long odds, the jury found Mr. and Mrs. Collins guilty as charged. But did they make the right decision? Was the mathematician's calculation correct? Malcolm Collins said it was not, and appealed his conviction.

In 1968, the Supreme Court of the State of California handed down a 6-to-1 decision, and their written opinion has become a classic in the study of legal evidence. Generations of law students have studied the case as an example of the use (and misuse) of mathematics in the courtroom. (As you will see, it's worrying that one supreme court justice reached a different conclusion, but at least the system worked.)

The justices said, among other things:

"We deal here with the novel question whether evidence of mathematical probability has been properly introduced and used by the prosecution in a criminal case. ... Mathematics, a veritable sorcerer in our computerized society, while assisting the trier of fact in the search for truth, must not cast a spell over him. We conclude that on the record before us defendant should not have had his guilt determined by the odds and that he is entitled to a new trial. We reverse the judgment. ..."

The Supreme Court's devastating deconstruction of the prosecution's "trial by mathematics" had three major elements:

Proper use of "math as evidence" versus improper use ("math as sorcery"). Failure to prove that the mathematical argument used actually applies to the case at hand. A major logical fallacy in the prosecutor's claim about the extremely low chance of the defendants being innocent.

Math as evidence

The Supreme Court believed that the appeal of the "mathematical conclusion" of odds of 1 in 12 million was likely to be too dazzling in its apparent "scientific accuracy" to be discounted appropriately in the usual weighing of the reliability of the evidence. This, they opined, was mathematics as sorcery.

Was the trial court's math correct?

But I doubt that any regular reader of this column would be so foolish as to assume that the six features the prosecutor gave the expert witness came even close to being independent.

Was the prosecutor's claim valid?

On the one hand, there is the prosecution's calculation, which in spite of its lack of justification, attempts to determine

P(match) = the probability that a random couple would possess the distinctive features in question (bearded black man, with a mustache, etc.)

P(innocence) = the probability that Mr. and Mrs. Collins are innocent.

As the justices explained in their opinion, a "probability of innocence" calculation (even if one could presume to actually calculate such a thing) has to take into account how many other couples in the Los Angeles area also have those six characteristics. The court said, "Of the admittedly few such couples, which one, if any, was guilty of committing the robbery?"

The court went on to perform another calculation: Assuming the prosecution's 1 in 12 million result, what is the probability that somewhere in the Los Angeles area there are at least two couples that have the six characteristics as the witnesses described for the robbers? The justices calculated that probability to be over 40 percent. Hence, it was not at all reasonable, they opined, to conclude that the defendants must be guilty simply because they have the six characteristics in the witnesses' descriptions.

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