Concord (Ps) include twenty-four corporations that manufacture and sell recreational boats. Ps brought this antitrust action against stern drive engine manufacturer D for violations of Sections 1 and 2 of the Sherman Act, and §§ 1 and 2 of Section 7 of the Clayton Act. Manufacturers use standard automobile engine blocks to make stern drive engines for motor boats. They 'marinize' the automobile engines and equip them with a drive system and then sell them to boat builders who may be affiliated with the manufacturer or may be independent buyers. Stern drive engines are used primarily in recreational powerboats known as runabouts, which are typical water skiing boats, and in cruising boats, which are larger and more expensive boats and usually have cabins. Runabouts and cruising boats together make up about 40% of all recreational power boats. D has been the market leader in stern drive engine manufacturing for many years. In 1983 it had earned a 75% market share. In 1984, D offered market share discounts to boat builders and dealers. If boat builders and dealers agreed to purchase a certain percentage of their engine requirements from D for a fixed period of time they would get a discount off the list price of the engine. D offered a 3% discount for an 80% market share of their engines, a 2% discount for 70% and a 1% discount for those who took 60% of their needs from D The level for discounts was lowered, and also an additional 1-2% was offered for an additional 2-3 years. The market share discounts were eliminated entirely in the middle of 1997. A new engine was introduced by a competitor which made significant market share impact upon D. D decided to buy two of the largest boat builders in the hope that this vertical integration would result in higher quality and a less expensive product. The new engine was a dud as all of them were recalled, and D was once again back on top. Ps filed this suit in 1995. Ps alleged that D had violated Section 7 of the Clayton Act by acquiring Bayliner and Sea Ray and that D had used its market share discounts, volume discounts, and long-term discounts and contracts, coupled with the market power it had achieved in purchasing Bayliner and Sea Ray, to restrain trade and to monopolize the market in violation of Sections 1 and 2 of the Sherman Act. Ps' primary evidence to establish antitrust liability was presented by Dr. Hall, a professor of economics at Stanford University. He testified that D had monopoly power in the stern drive market that enabled it to use its market share discount programs to impose a 'tax' on boat builders and dealers who chose to purchase engines from other manufacturers. Dr. Hall relied on the Cournot model of economic theory that posits that a firm 'maximizes its profits by assuming the observed output of other firms as a given, and then equating its own marginal cost and marginal revenue on that assumption.' Under this theory, any market share over 50% would be evidence of anticompetitive conduct on D's part. Dr. Hall concluded that D had engaged in anticompetitive conduct and that the boat builders had been overcharged at the moment D's market share surpassed the 50% threshold. D moved to exclude Dr. Hall’s testimony as speculation. The court refused. D was found guilty of violating three antitrust statutes. and D appealed. D, in part, claims that Dr. Hall's testimony should not have been admitted because his approach did not provide a reasonable basis for either liability or damages.