How math can turn an acquittal into a guilty verdict

From The New York Times:

ITALY’S highest court on Tuesday overturned the acquittal of Amanda Knox, accused of the 2007 murder of Meredith Kercher, a 21-year-old British woman who was Knox’s roommate in Perugia, Italy, at the time.

In 2011, an appeals court invalidated the 2009 murder convictions of Ms. Knox, an exchange student from Seattle, and her former boyfriend, Raffaele Sollecito, and released them from jail. Now, Italy’s Court of Cassation has annulled that decision — sending a strange new ripple into a case that has riveted many around the globe for years.

There is enough sensational detail in the Knox case, of course, to keep the tabloid pages filled. At least 10 books have been written about it, including one by Ms. Knox herself, whose memoir is to be published next month. But one aspect of this case — as with so many others, sadly — deserves far more attention than it gets: much unnecessary drama has resulted from bad math.

Miscalculation by judges and lawyers of probabilities, from the odds of DNA matches to the chance of accidental death, have sent innocent people to jail, and, perhaps, let murderers walk free.

The Court of Cassation has not yet publicly explained the motivations behind its ruling. But the appellate judge’s failure to understand probability may well play a role.

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