In Expert Witnesses and Reports, Manuel Lopez and David Chaumette of Shook, Hardy & Bacon L.L.P. write on when the expert’s facts are wrong:
An expert’s opinion is unreliable if his or her facts are wrong. As the Supreme Court explained, if “the foundational data underlying the opinion is unreliable, … any opinion drawn from that data is likewise unreliable.” Havner, 953 S.W.2d at 714. One variant of this argument is when the expert makes unwarranted assumptions. For example, the Seventh Circuit affirmed the exclusion of a “lost profits” expert on the basis that his assumptions about market penetration were “optimistic.” Target Market Pub., Inc. v. ADVO, Inc., 136 F.3d 1139, 1144 (7th Cir. 1998). Target Market is significant because the expert’s assumptions were similar to the defendant’s own projections of potential profits in its marketing plans. Id. The Seventh Circuit held that experts still needed to prove the assumptions behind such projections. See Id. at 1145 (“The [marketing] plan sought to demonstrate what Select Auto’s profits might be given certain assumptions that had not yet, and might never, come to pass.”).