People v. Collins

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People v. Collins[1] was a 1968 American robbery trial noted for its misuse of probability[2] and as an example of the prosecutor's fallacy.[3][4]


After a mathematics instructor testified about the multiplication rule for probability, though ignoring conditional probability, the prosecutor invited the jury to consider the probability that the accused (who fit witness' description of a black male with a beard and moustache, and a caucasian female with a blonde ponytail, fleeing in a yellow car) were not the robbers, suggesting that they estimate:

Black man with beard   1 in 10
Man with mustache 1 in 4
White woman with pony tail 1 in 10
White woman with blonde hair 1 in 3
Yellow motor car 1 in 10
Interracial couple in car 1 in 1000

The jury returned a guilty verdict.


The California Supreme Court set aside the conviction, criticising the statistical reasoning for ignoring dependencies between the characteristics e.g. bearded men commonly sport moustaches. The court asserted that mathematics,

while assisting the trier of fact in the search of truth, must not cast a spell over him.


  1. ^ People v. Collins, 68 Cal.2d 319, 438 P.2d 33 (1968)
  2. ^ Laurence H. Tribe, Trial by Mathematics: Precision and Ritual in the Legal Process, 84 HARV. L. REV. 1329 (1971).
  3. ^ Michael O. Finkelstein & William B. Fairley, A Bayesian Approach to Identification Evidence, 83 HARV. L. REV. 489 (1970).
  4. ^ Kreith, K. (1976) "Mathematics, Social Decisions and the Law", International Journal of Mathematical Education in Science and Technology vol.7 p315


  • Leila Schneps and Coralie Colmez, Math on trial. How numbers get used and abused in the courtroom, Basic Books, 2013. ISBN 978-0-465-03292-1. (Second chapter: "Math error number 2: unjustified estimates. The case of Janet Collins: hairstyle probability").

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