Friday, October 21, 2011

A Lesson in Probability by the Massachusetts Supreme Judicial Court


A slight shudder goes over me when I realize that I am about to encounter another judicial disquisition on probability and factual inference.

I will give you the citation first, and I will discuss the opinion in a later post or posts: Commonwealth v. Ferreira, No. SJC-10902, 2011 Mass. LEXIS 977 (Oct. 21, 2011).

But I feel compelled to make very brief comment on two points now:

1. The case involved a pretrial identification. The prosecutor made an off-the-cuff argument about the improbability that the victim would have picked, out of two photo arrays containing a total of 14 people, two suspects who happened to know each other. Whether or not the court got the final result right, the court failed to understand the point of the prosecutor's argument about random selection.

2. Whether or not the court got the final result right, the court -- like practically every other court before it -- parroted vacuous and misleading cliches about the non-mathematical character of the reasonable doubt standard:
The prosecutor also erred in equating proof beyond a reasonable doubt with a numerical percentage of the probability of guilt, in this case, ninety-eight per cent. "[T]o attempt to quantify proof beyond a reasonable doubt changes the nature of the legal concept of 'beyond a reasonable doubt,' which seeks 'abiding conviction' or 'moral certainty' rather than statistical probability." Commonwealth v. Rosa, 422 Mass. 18, 28 (1996). "The idea of reasonable doubt is not susceptible to quantification; it is inherently qualitative." Commonwealth v. Sullivan, 20 Mass. App. Ct. 802, 806 (1985). See Commonwealth v. Mack, 423 Mass. 288, 291 (1996) ("the concept of reasonable doubt is not a mathematical one").
Compare P.Tillers & J. Gottfried, United States v. Copeland: A Collateral Attack on the Legal Maxim that Proof Beyond a Reasonable Doubt Is Unquantifiable?, 5 Law, Probability and Risk 135 (Oxford University Press, 2006).
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