Detecting Non-Adversarial Collusion in Crowdsourcing

A group of agents are said to collude if they share information or make joint decisions in a manner contrary to explicit or implicit social rules that results in an unfair advantage over non-colluding agents or other interested parties. For instance, collusion manifests as sharing answers in exams, as colluding bidders in auctions, or as colluding participants (e.g., Turkers) in crowd sourcing. This paper studies the latter, where the goal of the colluding participants is to “earn” money without doing the actual work, for instance by copying product ratings of another colluding participant, adding limited noise as attempted obfuscation. Such collusion not only yields fewer independent ratings, but may also introduce strong biases in aggregate results if undetected. Our proposed unsupervised collusion detection algorithm identifies colluding groups in crowd sourcing with fairly high accuracy both in synthetic and real data, and results in significant bias reduction, such as minimizing shifts from the true mean in rating tasks and recovering the true variance among raters.