From Sommerfield v. City of Chicago, 2008 U.S. Dist. LEXIS 88760 (N.D. Ill. Nov. 3, 2008):

Acceptance of the notion that an expert can reasonably base his opinion on summaries of deposition testimony prepared by a party's lawyer would effectively eliminate Daubert 's insistence that an expert's opinion be grounded on reliable information. This conclusion follows from a number of separate but related principles, perhaps the most critical of which is the partisan role lawyers play in our adversary system. Indeed Judge Posner has said that'" [expert witnesses] are the mere paid advocates or partisans of those who employ and pay them, as much so as the attorneys who conduct the suit.'" Olympia Equipment Leasing Co. v. Western Union Telegraph Co., 797 F.2d 370, 382 (7th Cir. 1986).... There is nothing improper about this partisanship. Quite the contrary. Neutrality is for the court, not for those actively engaged in the struggle, and a lawyer representing a party in litigation is expected to be a zealous advocate on behalf of a client to whom is owed undivided allegiance....

Consistent with the role of an advocate in our adversary system, it is assumed that counsel "is supposed to give the evidence a partisan slant." Philips Medical Systems Intern. B.V. v. Bruetman, 8 F.3d 600, 606 (7th Cir. 1993)(Posner, J.). I ndeed, "a partisan scrutiny of the record and assessment of potential issues, goes to the irreducible core of the lawyer's obligation to a litigant in an adversary system...." Smith v. Robbins, 528 U.S. 259, 293 (2000)(Souter, J., dissenting). This single-minded devotion to a client's interests — which "follows from the nature of our adversarial system of justice," Penson v. Ohio, 488 U.S. 75, 84 (1988) — is incompatible with the neutrality and evenhandedness that are necessary if a summarization of deposition testimony is to have the reliability Daubert demands....

Moreover, since lawyers are not experts in the particular field involved in the case, their summarizations of deposition testimony run the risk of under-inclusion; that is, of inadvertent exclusion (or minimization) of information whose importance might elude the lawyer, but to the expert's trained eye might have spoken volumes — if only it had been included in the summary. Indeed, the very act of summarization of depositions by a party's lawyer to be used as the basis for expert testimony poses risks to the reliability that Daubert and Kumho Tire require.... [I]f a summary is based on the transcript, there is no justification for not supplying the deposition transcript, itself.***

To presume the reliability of the plaintiff's lawyer's summarizations of deposition testimony — and allow Mr. Pastor's [expert] testimony to be based solely on those summaries — would be an abdication of the screening function Daubert and Kumho Tire have imposed on district courts.

Nonetheless, the expert was not barred from testifying at trial based on the testimony adduced at trial.

The testimony adduced at trial can be made known to Mr. Pastor either through listening to trial testimony or hypothetical questioning and could form the basis of his opinions. See Rule 703; United States v. Crabtree, 979 F.2d 1261, 1270 (7th Cir. 1992); Moore v. Ashland Chem., Inc., 126 F.3d 679, 690 (5th Cir. 1997). ***

The purpose of an expert report is to facilitate an effective cross-examination, minimize the expense of deposing experts, the shortening of direct examination, and the prevention of ambush at trial.... While theoretically out of the case, Mr. Pastor's report nonetheless provides sufficient information about Mr. Pastor's opinions to enable the City to take a comprehensive deposition. The City has asked for permission to depose Mr. Pastor, and fundamental fairness demands that it have that opportunity. This course is preferable to allowing Mr. Pastor to submit a new report, which I have the discretion to allow, and will enable the jury to have information that may help it to make a fully informed, merits-based decision, which after all is the desideratum of the whole endeavor. See Committee Note to 2000 Amendments to Rule 702("'Trial courts should be allowed substantial discretion in dealing with Daubert questions....'").