study. This type of challenge is novel in the social sciences but common in computer science, where teams are often invited to compete in tasks such as machine translation of a standard set of texts or detection of an animal in a large set of photographs. The achievement of the winning team in these competitions defines the state of the art at a point in time, which is always exceeded in the next competition. In a social science prediction task, where rapid improvement is not expected, it is reasonable to use the most accurate prediction achieved in the competition as a measure of the
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