A Geometric Proof of Calibration

  • Authors:
  • Shie Mannor;Gilles Stoltz

  • Affiliations:
  • Israel Institute of Technology (Technion), 32000 Haifa, Israel;École Normale Supérieure--CNRS--INRIA, 75005 Paris, France and HEC, Paris--CNRS, 78351 Jouy-en-Josas, France

  • Venue:
  • Mathematics of Operations Research
  • Year:
  • 2010

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Abstract

We provide yet another proof of the existence of calibrated forecasters; it has two merits. First, it is valid for an arbitrary finite number of outcomes. Second, it is short and simple and it follows from a direct application of Blackwell's approachability theorem to a carefully chosen vector-valued payoff function and convex target set. Our proof captures the essence of existing proofs based on approachability (e.g., the proof by Foster [Foster, D. 1999. A proof of calibration via Blackwell's approachability theorem. Games Econom. Behav.29 73--78] in the case of binary outcomes) and highlights the intrinsic connection between approachability and calibration.