In his judgment today in Merck Sharp and Dhome Limited -v- Shionig & Co Limited [2016] EWHC 2989 (Pat) Mr Justice Arnold made some observations about the cross-examination of expert witnesses. These related to experts in patent cases, they are also of more general interest.

“… in my experience too much time is spent by cross-examiners in patent cases on ad hominem attacks that are unfair to the witness, unhelpful to the court and waste expensive time.”


The cross-examination of expert witnesses in patent cases
  1. In a passage in Medimmume Ltd v Novartis Pharmaceuticals UK Ltd [2011] EWHC 1169 (Pat) at [99]-[114] which I believe is fairly well known, I considered the preparation of experts’ reports in patent cases. I explained that not only did expert witnesses owe the court a duty to be independent and impartial, but also the lawyers who assisted the experts to prepare their reports bore a heavy responsibility for ensuring that expert witnesses were not put in a position where they could be made to appear to have failed in their duty to the court even though they conscientiously believed that they had complied with that duty. I concluded by observing:
“It is also important that courts should be cautious about criticising an expert witness purely on the basis of omissions from his report unless it is clear that the fault lies with the expert rather than those instructing him, bearing in mind that the court will not usually be privy to the expert’s full instructions (whatever may be the effect of CPR r. 35.10(4), which it is not necessary to go into for present purposes).”
  1. What I did not explicitly say, but should have been clear, is that advocates who cross-examine expert witnesses in patent actions should also be cautious about criticising an expert witness purely on the basis of omissions from his (or her) report unless it is clear that the fault lies with the expert rather than those instructing him.
  2. More generally, in my experience too much time is spent by cross-examiners in patent cases on ad hominem attacks that are unfair to the witness, unhelpful to the court and waste expensive time. It is, of course, both legitimate and helpful to explore such matters as the witness’ qualifications and experience, and hence the extent to which the witness has relevant expertise and/or is representative of the skilled person; the basis upon which the witness considers that information was or was not common general knowledge (which may include whether the witness has correctly understood the concept); the witness’ approach to issues such as obviousness (which may include whether the witness has fully appreciated the need to avoid hindsight and whether he or she has actually avoided hindsight); and so on. But cross-examiners must refrain from using the fact that the expert has not mentioned something in their report as a stick to beat the witness with unless the cross-examiner has real grounds for suggesting that this reflects on the witness’ impartiality, competence or approach to the issues rather than upon the instructions they have been given. (If the witness has been wrongly instructed, that is, of course, a highly relevant matter, but it is a point for submissions rather than criticism of the witness.)
  3. A particular difficulty which often arises in multiple expert cases is the distribution of responsibilities between the experts. As noted above, the law recognises that some patents are addressed to skilled teams rather than skilled individuals. As the label implies, a skilled team is deemed to function as a team. In practice, however, it is extremely difficult for lawyers to instruct multiple experts as a team. The experts will often be busy people with limited availability, and whose schedules frequently do not coincide. Moreover, the steep learning curve for the lawyers in understanding the technical issues pushes them towards compartmentalisation. This means that, all too often, parties give insufficient attention to the interface between the different areas of expertise and between the different members of the expert team.
  4. This difficulty is exacerbated where there is an overlap between the expertises of the experts, as in the present case. In such circumstances the lawyers will usually be conscious of the need to avoid duplication between the experts’ reports. This will often lead to the lawyers deciding to ask one expert to deal with a particular topic even though another expert is equally well-placed to address it. Such decisions may be taken when the lawyers have an incomplete understanding of the issues, and may subsequently turn out to have been mistaken in the sense that it would have been better if the second expert had dealt with it.
  5. In such cases it may well be legitimate and helpful for the cross-examiner to explore the division of responsibility between the experts and the extent to which, collectively, they have approached matters in a manner which reflects the approach of the notional skilled team. What will rarely be legitimate or helpful is to criticise a witness for failing to deal with a point which he or she could have dealt with when it has been addressed by another expert, because this is unlikely to have been a decision made by the witnesses. Nor does it become any more legitimate or helpful if the cross-examiner either expressly asks the witness about the point or (more usually) asks the witness a question which leads the witness to bring it up.
  6. Finally, it should go without saying that cross-examiners should question experts fairly. In the present case experts on both sides were subjected to cross-examinations parts of which were in my view unfair even disregarding the points I have made above. Counsel for MSD expressed incredulity when Prof Tsantrizos said that she could not remember when she was first instructed and tackled her at some length on a couple of paraphrases of sentences in other reports which, while not completely accurate, were not materially inaccurate. Counsel for Shionogi intervened in the cross-examination to object to this line of questioning. Although it was not necessary for me to rule upon the objection at the time, since counsel for MSD was willing to move on, having considered the matter, I consider that the objection was well founded. Rightly, counsel for MSD did not pursue these criticisms in closing submissions. Counsel for Shionogi asked Prof Debyser a number of questions in the form “Did you consider the toxicity data in Hazuda [2000]?”. I intervened to stop this form of questioning because, apart from the fact that counsel had not taken the witness to the paper when asking the question, the point being put was that there was no toxicity data in it when the question implied that there were. (In fact, as Prof Debyser explained in re-examination, Hazuda 2000 did contain toxicity information; but that is a separate point.) Again, no criticism was made in closing submissions based on these questions.