DURA AUTOMOTIVE SYSTEMS OF INDIANA, INC., formerly known as Excel Corporation, Plaintiff-Appellant, v. CTS CORPORATION, Defendant-Appellee, 285 F.3d 609

POSNER, Circuit Judge. In 1981 the Main Street Well Field, part of the water supply of the city of Elkhart, Indiana, was discovered to be contaminated by TCE and other volatile organic compounds that are used as industrial or household solvents. Twelve years later the EPA, having cleaned up the contamination, sued several entities under the Superfund statute (Comprehensive Environmental Response, Compensation, and Liability Act of 1980, 42 U.S.C. §§ 9601 et seq.), including Dura Automotive Systems (actually its predecessor, Excel, but that’s a detail we can ignore), to recover the costs of the clean-up.

Dura impleaded CTS Corporation, claiming that it was responsible for some of the pollution and should therefore be required to reimburse a share of Dura’s clean-up expense. The EPA’s claims were settled or otherwise resolved, leaving only Dura’s third-party claim against CTS. At the behest of CTS and on the authority of Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579, 592-93, 113 S.Ct. 2786, 125 L.Ed.2d 469 (1993), the district judge disqualified Dura’s sole expert witness; and then, holding that the remaining evidence was insufficient to create a genuine issue of material fact, he granted

summary judgment for CTS. The groundwater stream beneath CTS’s plant, a plant that manufactures plastic by a process that employs volatile organic compounds, is 3,000 feet west of the Main Street Well Field and runs generally south rather than east. In contrast, Dura’s plant, which also uses these chemicals, is across the street from the well field and it was Dura rather than CTS that was a defendant in the EPA’s suit. But conceivably some of the groundwater beneath CTS’s plastics plant had seeped into the well field back in the late 1970s or early 1980s, contributing to the pollution discovered in 1981.

This could only be so, however, if CTS’s plant was within the well field’s “capture zone” (in other words, its catchment basin), the area within which groundwater, if present, could be expected to flow to the well field. The size of the capture zone would depend on such things as the porosity of the soil and — a factor particularly emphasized by the parties-— the rate at which the well field pumps water. The more it pumps, the larger the capture zone, because the removal of groundwater beneath the field causes groundwater to be drawn in by gravity from other areas.

To use porosity, pumping, and other data that bear on the size of the capture zone to map the zone, hydrogeologists build mathematical models consisting of systems of equations that show changes in the boundaries of the zone as functions of changes in the causal variables, such as porosity. These models are used to predict the future size of capture zones but they can also be used to estimate the size of a capture zone in the past if the requisite historical data on the causal variables are available, here, for example, data on pumping rates in the late 1970s.

The parties agree that without such a model Dura could not prove its case against CTS. A consulting firm retained by the EPA in the original suit had placed CTS’s plant outside the well field’s capture zone, and if this placement stood, CTS could not have been a source of the pollution of the Main Street Well Field and so Dura would have no right of reimbursement by CTS. Dura designated as its one and only expert witness Nicholas Valkenburg, a hy-drogeologist who works for a consulting firm called Geraghty & Miller.

At his deposition, however, Valkenburg admitted that he was not an expert in mathematical models of groundwater flow and that the modeling on which he relied for his conclusion that CTS’s plastics plant was indeed within the well field’s capture zone had been done by other employees of Geraghty & Miller, using two models, QuickFlow and SLAEM. When CTS moved that Valkenburg be barred from testifying and that Dura’s third-party claim be dismissed, Dura responded with affidavits from four employees or ex-employees of Geraghty & Miller who had worked on the Dura project.

These professional groundwater-flow modelers attested that the models they had used, QuickFlow and SLAEM, were reliable and were appropriate for determining the well field’s capture zone in the late 1970s. CTS moved to strike the affidavits under Fed.R.Civ.P. 37(c)(1) on the ground that Dura’s disclosure of additional expert witnesses, required by Rule 26(a)(2), was untimely, since the deadline for filing expert reports had expired six months previously.

The district judge granted the motion to strike, and holding that without the affidavits there was insufficient evidence of the reliability of the models, ruled that Valkenburg could not testify — and without Valkenburg’s testimony, Dura had no case. So the judge granted

summary judgment for CTS. If the affidavits were properly struck, Valkenburg’s testimony indeed lacked an adequate foundation. For while there is a smattering of other evidence about the models (for example, that they are widely used), it does not establish their appropriateness for mapping the well field’s capture zone 20 odd years ago.

Dura argues, however, that even if properly struck as untimely expert witness reports, so that Dura would not be able to call the authors as expert witnesses at trial, the affidavits remained usable as evidence that the QuickFlow and SLAEM models were indeed suitable for the purpose to which Valkenburg sought to put them. This is true to the extent that the affidavits merely attest to facts or opinions on which Valkenburg would be permitted to rely, such facts as that SLAEM had been peer reviewed.

But to the extent that the affidavits contain evidence that would have to be presented at trial by an expert witness or witnesses other than Valkenburg in order for Dura to withstand a motion for judgment as a matter of law, Dura’s failure to have made timely disclosure of their expert opinions invited application of Rule 37(c)(1) to bar the authors of the affidavits, or any other expert for that matter, from testifying along with Valkenburg. NutraSweet Co. v. X-L Engineering Co., 227 F.3d 776, 785-86 (7th Cir.2000); Salgado by Salgado v. General Motors Corp., 150 F.3d 735, 741 n. 6 (7th Cir.1998); Lohnes v.

Level 3 Communications, Inc., 272 F.3d 49, 60-61 (1st Cir.2001). We must decide whether the district judge was reasonable in regarding the affidavits as experts’ reports that Dura had failed to disclose to CTS in a timely fashion rather than merely as attestations that show that Valkenburg was competent to report the results of the modeling exercises undertaken by other employees of the consulting firm. As the form in which we have stated the question makes clear, we must give the benefit of the doubt to the district judge. General Electric Corp. v.

Joiner, 522 U.S. 136, 141-43, 118 S.Ct. 512, 139 L.Ed.2d 508 (1997); NutraSweet Co. v. X-L Engineering Co., supra, 227 F.3d at 786; Yeti by Molly, Ltd. v. Deckers Outdoor Corp., 259 F.3d 1101, 1106 (9th Cir.2001). An expert witness is permitted to use assistants in formulating his expert opinion, and normally they need not themselves testify. United States v. Bramlet, 820 F.2d 851, 855-56 (7th Cir.1987); United States v. Lawson, 653 F.2d 299, 301-02 (7th Cir.1981).

The opposing party can depose them in order to make sure they performed their tasks competently; and the expert witness can be asked at his deposition whether he supervised them carefully and whether his relying on their assistance was standard practice in his field. If the requisite assurances are forthcoming, the assistants’ work need not be introduced into evidence. Rule 703 of the Federal Rules of Evidence is explicit that “the facts or data in the particular case upon which an expert bases an opinion or inference may be those perceived by or made known to the expert at or before the hearing.

If of a type reasonably relied upon by experts in the particular field in forming opinions or inferences upon the subject, the facts or data need not be admissible in evidence in order for the opinion or inference to be admitted.” Analysis becomes more complicated if the assistants aren’t merely gofers or data gatherers but exercise professional judgment that is beyond the expert’s ken. (They needn’t, of course, be assistants.

We use the term because that seems the best description of the relation of the four affiants to Valkenburg, but it would make no difference if they were independent experts.) Now it is common in technical fields for an expert to base an opinion in part on what a different expert believes on the basis of expert knowledge not possessed by the first expert; and it is apparent from the wording of Rule 703 that there is no general requirement that the other expert testify as well.

The Committee Notes to the 1972 Proposed Rule 703 give the example of a physician who, though not an expert in radiology, relies for a diagnosis on an x-ray. We too do not “believe that the leader of a clinical medical team must be qualified as an expert in every individual discipline encompassed by the team in order to testify as to the team’s conclusions.” Walker v. Soo Line R.R., 208 F.3d 581, 589 (7th Cir.2000); see also United States v. Smith, 869 F.2d 348, 355 (7th Cir.1989); Ferrara & DiMercurio v. St. Paul Mercury Ins. Co., 240 F.3d 1, 8-9 (1st Cir.2001).

But suppose the soundness of the underlying expert judgment is in issue. Suppose a thoracic surgeon gave expert evidence in a medical malpractice case that the plaintiffs decedent had died because the defendant, a radiologist, had negligently failed to diagnose the decedent’s lung cancer until it was too advanced for surgery.

The surgeon would be competent to testify that the cancer was too advanced for surgery, but in offering the additional and critical judgment that the radiologist should have discovered the cancer sooner he would be, at best, just parroting the opinion of an expert in radiology competent to testify that the defendant had x-rayed the decedent carelessly.

The case would be governed by our decision in In re James Wilson Associates, 965 F.2d 160, 172-73 (7th Cir.1992), where the issue was the state of repair of a building and “the expert who had evaluated that state — the consulting engineer — was the one who should have testified. The architect [the expert who did testify] could use what the engineer told him to offer an opinion within the architect’s domain of expertise, but he could not testify for the purpose of vouching for the truth of what the engineer had told him — of becoming in short the engineer’s spokesman.” See also TK-7 Corp. v.

Estate of Barbouti, 993 F.2d 722, 732 (10th Cir.1993). It is the same here. Valkenburg could have testified that the well field was contaminated by volatile organic compounds and that if CTS’s plastics plant was within the well field’s capture zone some of the contamination may have come from that plant. It does not follow that he could testify that the plant was within the well field’s capture zone. The Daubert test must be applied with due regard for the specialization of modern science.

A scientist, however well eredentialed he may be, is not permitted to be the mouthpiece of a scientist in a different specialty. That would not be responsible science. A theoretical economist, however able, would not be allowed to testify to the findings of an econometric study conducted by another economist if he lacked expertise in econometrics and the study raised questions that only an econometrician could answer. If it were apparent that the study was not cut and dried, the author would have to testify; he could not hide behind the theoretician.

We must decide whether the district judge was reasonable in concluding that this was such a case. The answer lies in the four affidavits of the Geraghty & Miller employees who did the modeling that mapped the capture zone. The affidavits are much alike, and we can confine attention to those of James Rumbaugh, who developed the QuickFlow model, and of Erie Evans, a current employee of Geraghty & Miller. Rumbaugh’s affidavit confirms that QuickFlow is a system of equations for mapping a capture zone from data causally related to the size of the zone.

It acknowledges that modeling groundwater flow (the capture zone being, remember, the area from which the groundwater that polluted a given site could have flowed) “is inherently not the most precise of scientific tools” because “one never possesses complete geotechnical information.” (So this is not like taking an x-ray.) As a result, “the process of constructing a valid and useful groundwater model is an iterative process that requires the exercise of sound technical judgment in evaluating all available geotechnical data to determine what input values should be used with respect to each parameter utilized in the model.” (In other words, professional discretion — expertise — is involved.) Rumbaugh goes on to say that in his experience “most hydro-geologists are not experts in modeling”-— and we know that Valkenburg is not; and groundwater modeling is not the sort of thing that a lab technician or other subprofessional does.

Rumbaugh therefore “generally work[s] with other hydrogeologists [such as Valkenburg] to provide modeling results upon which they can rely.” Rum-baugh states that he is familiar with SLAEM and while it is not as sophisticated as some groundwater models, it is “entirely adequate and appropriate” for answering the sort of question posed by the present case. That of course is an expert opinion. It is also his expert judgment (not Valkenburg’

s) that “it was reasonable and appropriate to use QuickFlow” in the case. He calibrated the QuickFlow model, that is, compared the results it generated with observable results for the current period (obviously, the accuracy of its map of the capture zone in the late 1970s cannot be observed).

He attests that the kind of calibration he did, though visual rather than quantitative, is “an accepted practice in the modeling community.” Evans’s affidavit adds that it is his “opinion today that the hydrogeologic data set available to Geraghty & Miller was adequate for the development of a groundwater system model that would provide reliable information regarding the area encompassed by the Well Field capture zone under certain conditions” and specifically that SLAEM “is a valid tool for determining the extent of the Well Field ‘capture zone’ at various times in the past,” reflecting “reasonable technical judgments” by himself, the other affiants, and Valken-burg.

He explains why SLAEM didn’t have to be recalibrated and attests that “the use of groundwater models to simulate historic conditions (‘backward-modeling’) is a valid and a commonly applied approach” and that “an inherent assumption” of SLAEM, namely “that groundwater flow is two-dimensional,” is “valid within the area surrounding the [Main Street] Well Field.” Obviously, these are expert opinions, not recitations of cut-and-dried procedures.

It is apparent from these affidavits that Valkenburg’s assistants did not merely collect data for him to massage or apply concededly appropriate techniques in a concededly appropriate manner, or otherwise perform routine procedures, and that he himself lacks the necessary expertise to determine whether the techniques were appropriately chosen and applied. Remember that there were two crucial issues — the map of the capture zone and whether, if CTS’s plant was within it, how much if any of the contamination of the well field was due to the groundwater running beneath that plant.

Valkenburg was not competent to opine on the first issue, and without an expert opinion on that issue Dura could not get to the second and so could not prevail. We are not hydrogeologists, but we can imagine that the assertion that visual calibration is an adequate substitute for exact measurement, or that groundwater flow is “two dimensional” rather than three dimensional, would be controversial in the relevant community of experts.

More important than these conjectures is Rum-baugh’s reference to the construction of a groundwater-flow model as an “iterative process,” a fancy way of describing tinkering with the original model until it yields satisfactory results. There is nothing wrong with such tinkering. But we must be realistic about expert evidence: Ger-aghty & Miller was hired to provide evidence favorable to Dura; so any margin of discretion in the construction of the groundwater-flow model could be expected to be exploited to Dura’s benefit.

That discretion was exercised not by Valken-burg but by Rumbaugh and the other affi-ants, for it was they who constructed the model, and the “iterative process” by which they did so is beyond the scope of Valkenburg’s expertise. The quotations that we gave from their affidavits show the breadth of the expert discretion that they exercised. Without their testimony explaining and justifying the discretionary choices that they made, his testimony would have rested on air.

Had Dura merely wanted to use SLAEM and QuickFlow to determine the current capture zone of the Elkhart well field, we might well have a different case; such use might be quite routine. Dura wanted to use these models to determine the capture zone twenty years ago. The affidavits make clear that adapting the models to that use required a host of discretionary expert judgments for the affi-ants, not Valkenburg, to make.

Dura argues that it could not have foreseen that the judge would find Valkenburg unqualified to give expert testimony in this case and therefore it should have been forgiven the untimely filing of additional experts’ reports. Rule 37(c)(1) states that expert testimony may not be presented at trial if the expert’s report was not disclosed to the other side within the deadline unless the party was justified in missing the deadline or the untimeliness of the disclosure was harmless. As some cases fail to note, however, see Miksis v. Howard, 106 F.3d 754, 760 (7th Cir.1997); Mid-America Tablewares, Inc. v.

Mogi Trading Co., 100 F.3d 1353, 1363 (7th Cir.1996); Wilson v. Bradlees of New England, Inc., 250 F.3d 10, 20-21 (1st Cir.2001), the rule goes on to authorize the judge, “in lieu of this sanction ... [to] impose other appropriate sanctions,” and this authorization implies, as United States v. Johnson, 228 F.3d 920, 926 (8th Cir.2000), rightly holds, that a judge’s failure to impose a lighter sanction might be an abuse of discretion even if the party could not show that missing the deadline was justified or harmless. Cf. Sherrod v. Lingle, 223 F.3d 605, 612-13 (7th Cir.2000). But this is not such a case.

Not only was there no justification for not disclosing to CTS the opinions of the other experts — for Dura should have known that Valken-burg’s expertise did not extend to scientific issues at once crucial to the prima facie case and likely to be contested. In addition, the suit was in its seventh year when the judge acted; to have reopened discovery to give CTS its crack at the additional experts would have extended the litigation, and burdened CTS, unreasonably. Dura is a substantial firm rather than a hapless individual. Its reticence about disclosing the other experts may have been strategic.

At all events, in the circumstances the district judge could refuse to exercise lenity without being thought to have acted unreasonably. Affirmed.