The Laws of Probability

Around noon one day last June, an elderly woman was mugged in an alley in San Pedro, Calif. Shortly afterward, a witness saw a blonde girl, her pony tail flying, run out of the alley, get into a yellow car driven by a bearded Negro, and speed away. Police eventually arrested Janet and Malcolm Collins, a married couple who not only fitted the witness's physical description of the fugitive man and woman but also owned a yellow Lincoln. The evidence, though strong, was circumstantial. Was it enough to prove the Collinses guilty beyond a reasonable doubt?

Confidently answering yes, a jury has convicted the couple of second-degree robbery because Prosecutor Ray Sinetar, 30, cannily invoked a totally new test of circumstantial evidence--the laws of statistical probability.

In presenting his case, Prosecutor Sinetar stressed what he felt sure was already in the jurors' minds: the improbability that at any one time there could be two couples as distinctive as the Collinses in San Pedro in a yellow car. To "refine the jurors' thinking," Sinetar then explained how mathematicians calculate the probability that a whole set of possibilities will occur at once. Take three abstract possibilities (A, B, C), and assign to each a hypothetical probability factor. A, for example, may have a probability of 1 out of 3; B, 1 out of 10; C, 1 out of 100. The odds against A, B and C occurring together are the product of their total probabilities (1 out of 3 X 10 X 100), or 3,000 to 1.

After an expert witness approved Sinetar's technique, the young prosecutor asked the jury to consider the six known factors in the Collins case: a blonde white woman, a ponytail hairdo, a bearded man, a Negro man, a yellow car, an interracial couple. Then he suggested probability factors ranging from 1-to-4 odds that a girl in San Pedro would be blonde to 1-to-1,000 odds that the couple would be Negro-white. Multiplied together, the factors produced odds of 1 to 12 million that the Collinses could have been duplicated in San Pedro on the morning of the crime.

Public Defender Donald Ellertson strenuously objected on grounds that the mathematics of probability were irrelevant, and that Sinetar's probability factors were inadmissible as assumptions rather than facts. Sinetar, however, merely estimated the factors before inviting the jurors to substitute their own. And the public defender will not appeal because he found no trial errors strong enough to outweigh the strong circumstantial evidence. Convicted by math, Malcolm Collins received a sentence of one year to life. Janet Collins got "not less than one year."

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