If, however, you check the fine print and notice that all four newspapers got the story from the Associated Press, you go back to suspecting it’s a prank, this time by an AP reporter. Rule systems have no way of dealing with this, and neither does Naïve Bayes. If it uses features like Reported in the New York Times as predictors that a news story is true, all it can do is add Reported by AP, which only makes things worse. The breakthrough came in the early 1980s, when Judea Pearl, a professor of computer science at the University of California, Los Angeles, invented a new representation:
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