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In antitrust cases brought against cartels, the but-for price is an estimate of the price that would have prevailed in the absence of there being a conspiracy designed to elevate prices above what they otherwise would have been.
In US civil antitrust cases, one sees damages estimates being constructed by taking the difference between the actual price and the estimated but-for price and multiplying by the quantity purchased. To estimate a but-for price, one approach is to construct an econometric model that reliably predicts price variation during a benchmark period in which it is reasonable to assume conduct was not collusive, and then to use this model to predict what prices would have been during the period of collusion. For example, economists might use a predictive econometric model that accounts for demand and cost factors relevant for the product, market, and industry under consideration.