How "Speculative" May Expert Testimony Admissibly Be?

In a suit by HMOs to recover medical costs incurred by their members due to misleading cigarette marketing, the Eighth Circuit has upheld the exclusion of an economist's testimony comparing the incidence of health problems in the real world versus a "counterfactual" world in which the cigarette companies didn't conspire to mislead the public. See Group Health Plan, Inc. v. Philip Morris USA, Inc., No. 02-1684 (8th Cir. Sept. 16, 2003) (Arnold, Hansen, & Reid, JJ.). The trial court excluded the testimony as speculative. The appellate panel affirmed but seemed slightly impatient with the idea that "speculation" was necessarily fatal to an expert's testimony: "Although courts cast their assessment of how much speculation is permissible in various verbal forms, their conclusions in cases involving counterfactual estimations essentially come down to this: A certain amount of speculation is necessary, an even greater amount is permissible (and goes to the weight of the testimony), but too much is fatal to admission."

As Judge Arnold's opinion goes on to note, that nostrum may not be too helpful in deciding individual cases. But it's refreshing to see a judicial admission that "speculation" is often an inevitable element of expert opinion.