A shooting last weekend at a children’s birthday party in California that left four dead was the 17th mass killing this year — the lowest number recorded since 2006, according to a database maintained by The Associated Press and USA Today in partnership with Northeastern University.

Experts warn that the drop doesn't necessarily mean safer days are here to stay and that it could simply represent a return to average levels.

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“Sir Isaac Newton never studied crime, but he says ‘What goes up must come down,'" said James Alan Fox, a criminologist at Northeastern University. The current drop in numbers is more likely what statisticians call a “regression to the mean,” he said, representing a return to more average crime

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