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June 25, 2008

The Psychology of Fuel Efficiency

A recent discussion started by John Lynch on the Society for Judgment and Decisionmaking listserv focuses on an interesting new article by Larrick and Soll in Science, entitled the "MPG Illusion."  The paper reemphasizes the point that statistical metrics matter.  It argues that the traditional miles per gallon metric leads people to make inaccurate judgments on the benefits of more efficient cars.

For example, Richard Larrick in his podcast makes an argument along the following lines.  Say you have the ability to trade in a 10 MPG SUV for a 20 MPG crossover, or a 25 MPG car for a 50 MPG hybrid.  Which switch is better for the environment?  As it turns out, the former, even though one might be tempted to say that the former only improves efficiency by 10 MPG while the latter improves it by 25.   Assume a 100 mile trip.  The SUV will consume 10 gallons versus 5 gallons for the crossover for a net savings of 5 gallons.  The car will consume 4 gallongs versus 2 gallons for the hybrid for a net savings of 2 gallons. 

It seems that since we drive given distances (e.g. 100 mi), rather than specific amounts of fuel, the MPG is a misleading measure of efficiency.  Small increases in efficiency down at the low end make much more of a difference than at the high end.  Larrick & Soll argue that an inverse ratio, gallons per 10,000 miles, might be a more useful measure.

More information is available on Larrick's website, which has links to the Science article, podcast, and supplemental materials.


June 25, 2008 | Permalink | Comments (1) | TrackBack

June 24, 2008

The persuasive power of neuroscience

The March issue of the Journal of Cognitive Neuroscience contains an article stimulated by the  frequent appearance of news stories announcing the latest brain signature -- for love, aggression, greed, lying, etc. A group of researchers at Yale decided to investigate whether people can distinguish solid claims about these associations from poorly substantiated ones. The researchers wrote explanations for well-documented psychological phenomena. Some versions presented scientifically accepted rationales and sound reasoning. Other explanations were circular. People with no training in psychology or neuroscience distinguished the good from bad -- until an utterly irrelevant mention of the physical brain was added. The bad explanations became far more believable when they included a mention of neuroscience, while the good accounts got only a slight boost. People with advanced training in cognitive science were immune to this "seductive allure of neuroscience."

Does this finding have some bearing on the law's demand for validation of scientific evidence? Does it support a distinction between "soft" psychological testimony and testimony about brain imaging results?



Weisberg, D. S.; Keil, F. C.; Goodstein, J.; Rawson, E.; & Gray, J. (2008). The Seductive Allure of Neuroscience Explanations. Journal of Cognitive Neuroscience, 20(3), 470-477.

The description of the study is adapted from the May/June 2008 issue of the Yale Alumni Magazine, p. 38.

June 24, 2008 | Permalink | Comments (0) | TrackBack

June 22, 2008

Rounding Up the Usual Suspects III: People v. Nelson

On April 5, 2008, I mentioned People v. Nelson, 48 Cal.Rptr.3d 399 (Ct. App. 3 Dist. 2006), rev. granted, 147 P.3d 1011 (Cal. 2006), as a leading case on the admissibility of the various probabilities associated with cold hits in DNA databases. Last week, the California Supreme Court affirmed.

The case arose from the rape and murder of a nineteen-year-old college student in 1976. Dennis Nelson was a suspect, but the evidence was inconclusive, and the case grew cold. Later, Nelson was convicted of a different rape. His DNA profile was entered into the state convicted-offender databank. In 2001, investigators discovered that this profile matched those derived from stains from the 1976 rape. At that point, there were 184,000 profiles in the database. According to the state, the match would occur “at random among unrelated individuals in about one in 950 sextillion African-Americans, one in 130 septillion Caucasians, and one in 930 sextillion Hispanics.” As the court adds, “[t]here are 21 zeros in a sextillion and 24 zeros in a septillion.”

Nelson moved to dismiss the resulting charges on the ground that the delay between the 1976 crime and the charges filed in 2002 deprived him of his right to a speedy trial. The superior court denied the motion. At trial, Nelson conceded that he had intercourse with the victim but claimed that it was consensual -- somebody else must have murdered her and left her body in the mud. That did not work either. The jury convicted Nelson of first degree murder, and the Court of Appeal affirmed.

The California Supreme Court reviewed two claims. First, with respect to the speedy-trial issue, it held that the 26-year delay between the offense and the prosecution caused only slight prejudice and was justified.

Second, the court considered whether the vanishingly small random-match probabilities should have been admitted. The court correctly held that inasmuch as the procedure underlying this calculation was generally accepted and uncontested, the only real issue was the relevance of a random-match probability in a database-trawl case.

At this point, however, the opinion unravels. It contains but a single, short paragraph to show why the statistic is relevant:

        In a non-cold-hit case, we said that “[i]t is relevant for the jury to know that most persons of at least major portions of the general population could not have left the evidence samples.” (People v. Wilson, supra, 38 Cal.4th at p. 1245.) We agree with other courts that have considered the question (the Court of Appeal in this case; People v. Johnson, supra, 139 Cal.App.4th 1135; and Jenkins, supra, 887 A.2d 1013) that this remains true even when the suspect is first located through a database search. The database match probability ascertains the probability of a match from a given database. “But the database is not on trial. Only the defendant is.” (Modern Scientific Evidence, supra, § 32:11, pp. 118-119.) Thus, the question of how probable it is that the defendant, not the database, is the source of the crime scene DNA remains relevant. (Id. at p. 119.) The rarity statistic addresses this question.

As the co-author of the text of the treatise being quoted, I fear that these words are inconsistent with the portion of the court's opinion (in note 3) suggesting that the database-match probability also is relevant. If the issue is simply “how probable it is that the defendant, not the database, is the source of the crime scene DNA,” then the database-match probability is irrelevant. Unlike the “rarity statistic,” it does not figure into the probability that the named defendant is the source. The formulas are given and explained in a forthcoming article, Rounding Up the Usual Suspects: A Logical and Legal Analysis of DNA Trawling Cases. The Nelson court's theory that a variety of statistics are admissible in a database-trawl case does not withstand analysis. Or, if it does, it will take more analysis than this court has provided to explain why. The opportunity for such clarification may well arise, as there will some cases in which defense counsel will be interested in introducing the database-match probability, which can be orders of magnitude larger than the random-match probability.

In this particular case, however, the demand for an adjustment to the random-match probability is much ado about nothing. So what if the probability is 10–19 rather than 10–24? Having rejected the defense argument about general-acceptance, the court could simply have observed that the choice of a statistic could not have affected the outcome of the case. The court realized this, but it endorsed the 10–24 figure anyway.

-- DHK


People v. Nelson, No. S147051 (Cal. June 16, 2008), slip opinion available at http://www.courtinfo.ca.gov/opinions/documents/S147051.pdf

Dolan, Maura and Jason Felch. 2008. "California Supreme Court Ruling Allows 'Rarity' Statistic in DNA Cases." Los Angeles Times: June 17 available at  http://www.latimes.com/news/science/la-me-dna17-2008jun17,0,3313471.story

Kaye, David H. 2009. "Rounding Up the Usual Suspects: A Logical and Legal Analysis of DNA Trawling Cases". North Carolina Law Review: in press. Prepublication draft available at SSRN: http://ssrn.com/abstract=1134205

June 22, 2008 | Permalink | Comments (0) | TrackBack