@article{Neyman_Roughgarden_2022, title={Strictly Proper Contract Functions Can Be Arbitrage-Free}, volume={36}, url={https://ojs.aaai.org/index.php/AAAI/article/view/20449}, DOI={10.1609/aaai.v36i5.20449}, abstractNote={We consider mechanisms for truthfully eliciting probabilistic predictions from a group of experts. The standard approach --- using a proper scoring rule to separately reward each expert --- is not robust to collusion: experts may collude to misreport their beliefs in a way that guarantees them a larger total reward no matter the eventual outcome. It is a long-standing open question whether there is a truthful elicitation mechanism that makes any such collusion (also called "arbitrage") impossible. We resolve this question positively, exhibiting a class of strictly proper arbitrage-free contract functions. These contract functions have two parts: one ensures that the total reward of a coalition of experts depends only on the average of their reports; the other ensures that changing this average report hurts the experts under at least one outcome.}, number={5}, journal={Proceedings of the AAAI Conference on Artificial Intelligence}, author={Neyman, Eric and Roughgarden, Tim}, year={2022}, month={Jun.}, pages={5150-5155} }