16 May 2022

Expert Witnesses in Patent Cases – What’s Happening?

Since their appointments to the High Court bench in 2020 and 2021 respectively, Mr. Justice Meade and Mr. Justice Mellor have made a point of considering the use of expert evidence in patent cases. They have both, especially Meade J., commented on the value of expert evidence and provided guidance on the instruction of experts. Here we offer a summary of how the use of expert evidence is evolving.

The functions and duties of experts

The main function of an expert witness in a patent case is to assist the Court. An important difference between England and other jurisdictions is that there are clear rules on the impartiality of the expert with a focus on their duty to the Court ahead of pure client interest. This can surprise some people who are used to their experts being purely hired for their pro client views. The duty to the Court takes precedence over any duty to the party instructing and paying them, and they must focus on explaining the technology or process in question to the Court, and in particular, offer their views on the knowledge of the hypothetical “Skilled Person” to assist the Court in determining the “Common General Knowledge” (CGK).

How can they help and avoid hindsight?

In Philip Morris v BAT¹, Meade J. said that there is a difference between asking an expert witness the open question “What would the Skilled Team do?” and (for example) the closed question “Would it be obvious to use a gathered sheet of tobacco?”. The latter risks introducing hindsight, which ought to be avoided.

As mentioned in our article about Mellor J.’s decision in Teva v Bayer², when an expert is opining on the validity (especially in relation to obviousness) of a patent, it is extremely important that the expert view is not unduly affected by hindsight. Mellor J. said that hindsight can affect the obviousness case if, for example, an expert (knowing of the “target”) appears to be looking for ways to avoid taking a particular step or making a particular choice towards the target, when the Court concludes that the Skilled Person would consider the step or choice differently and, having considered it dispassionately, would decide to take the step or choice.

It is important for expert witnesses to concentrate on the real-world approach of the Skilled Team (comprising those skilled in the art or made up of Skilled Persons). They must explain how they believe a Skilled Team would likely undertake testing along several parallel paths. Mellor J. criticised the approach of Bayer’s expert, finding that his report placed too much emphasis on theory in hindsight, and this blighted the expert’s attempt to make it appear that the Skilled Team would not undertake the test in question. This approach was subsequently followed by Meade J. in Optis v Apple³.

The views of Meade J. build on the earlier work of Lord Justice Arnold when he sat in the High Court and offered guidance for the instruction of experts on avoiding hindsight in Medimmune v Novartis⁴. Arnold J. (as he then was) said that when the expert is instructed, the order in which they are taken through the matters and materials is important, and confirmed the approach in that case whereby the expert was first asked to consider the prior art, then the priority documents, and then the patents in question. This structure ensures the expert arrives at the patent with the relevant material and CGK in mind.

Meade J. agreed with this in broad terms in his decision in Fisher and Paykel v Flexicare⁵ where he observed that, in a perfect world, it would be best for experts to be asked about the CGK, then shown the prior art, and only then shown the patent in question, so that they would not be influenced by hindsight. However, it would seem there is a recognition that this is not always how instructions will be given, nor is it mandatory, because his wording suggests that while this is the best method for instruction, it is not a requirement. Indeed, in some cases it may not even be possible, as the expert might well already know what the invention of the patent in suit is, for example from the patentee’s commercial use of such⁶. Further, it is quite understandable that, when looking for an expert, early references to the patent to determine if the expert knows the subject area sufficiently to be an expert is understandable.

There are also times when this approach, of so-called “sequential unmasking” is unfeasible. Meade J. gave an example of such in Promptu v Sky⁷, noting that the patentee had maintained a number of dependent claims, and so Meade J. said it was inevitable that not all of them could be covered in the fullest depth in the expert’s first reports. He said that sequential unmasking was impossible after the first report, as the expert would at that point have knowledge of the patent in question before drafting the second (and, in this case, the third) reports. Meade J. took the potential for hindsight into account, but considered it a minor factor given the situation and the fact that the parties for the most part agreed on the CGK.

Meade J. also said that an expert could not be accused of hindsight just because their instructing solicitors had directed them to review specific parts of the prior art, or to discuss specific parts of the CGK in their report⁸. This is particularly the case where the alternative would lead to a long and rambling report which would not assist the Court. He suggested that the risk of hindsight was not necessarily fatal or even serious, as long as the expert acknowledges the pointer(s) that they received and provides evidence as to why that particular area of the prior art was of interest. In that particular case, the specific area of prior art in question was common ground between the parties.

Meade J. said⁹ that some directing of the expert was common sense, as the alternative would be unworkable. He said that it would not usually be sensible to ask an expert “What was the common general knowledge in your field at the time?” without some additional guidance, as that would lead to far more information being provided by the expert than would be relevant to the matter at hand.

So while it is acceptable to direct an expert to certain parts of the CGK or the prior art, there is also a risk in doing such if the expert misses something in the wider CGK or prior art. If an expert does not consider parts of either in their report, then they are open to cross-examination on those absences. As Arnold J. (as he then was) said in Medimmune¹⁰, if an expert thinks a particular passage is ambiguous and therefore best ignored, the solicitors instructing that expert should persuade them to include that passage in the report. If they do not, opposing counsel may well cross-examine the expert on the passage and potentially accuse them of failing in their duty to the Court.

Does the expert need the same precise experience as the Skilled Person?

In patent cases, there is a risk that a party will challenge the credentials of the expert witness for the other side and argue that they are not suitably qualified to provide expert evidence for certain parts of the case. Interestingly, Meade J. has been clear that approximation of the notional Skilled Person is not essential in an expert; as he said in Philip Morris¹¹, it is well established that the assessment of expert evidence in patent cases is not an exercise in determining which expert in fact approximates more closely to the notional Skilled Person. Indeed, it is not problematic that a specific expert may be cleverer or more inventive than the Skilled Person (and indeed may have themselves a number of patents in similar fields in their name), as long as they can put themselves in the shoes of the Skilled Person, or the “notional uninventive addressee”, as Meade J. said in Add2 v Dspace¹².

Nor is it necessarily an issue if the expert has less direct experience in the specific field but can still adequately present opinions on the notional Skilled Person in that field, whether by tangential experience or by reading relevant literature. As Meade J. said in Merck Sharp and Dohme Ltd v Wyeth LLC¹³, it is well established that the expert’s precise approximation to the notional Skilled Person is not the key question, so long as they are able to assist the Court to identify the CGK and the attitude and approach of the ordinary Skilled Person in their field. Although it can be noted that the opinions of experts tend to be given less weight when they do not have real-world experience of the matters in hand and there is limited literature to draw upon. For example, in Alcon v Aspire¹⁴, Meade J. said that he found one expert team to be more convincing because of one of the experts’ real-world experience with the subject matter in question. He said it was not the sole factor, but it was a contributory factor. He also commented on the challenges of the situation where the literature was limited in Add2¹⁵, and discussed how one of the experts was not in the field at the priority date but had attempted to put himself in the position of the notional Skilled Person by research and reading literature.

However, in that case, there was little published in the field. This meant, that while the judge commended the expert’s care and professionalism in finding out what he could, he thought that the expert could not be an authority on the mind-set of the Skilled Person at the time.

What counts as CGK?

In Teva UK Ltd v AstraZeneca AB¹⁶, Mr. Justice Sales confirmed that CGK includes statements in standard reference works and leading journal articles. In addition, he considered the recognition of reliability of the source was important, despite what some said at the time, and referred to a number of “primary articles” which he described as “academic articles…sufficiently prominent in the main academic journals in the field”, were part of the CGK. This starting place was not surprising, but he then described two further categories of CGK – “secondary articles” and “tertiary articles”. He described secondary articles as being a range of articles that were not leading journals in the field and would not have been likely to have been read by the notional Skilled Person in the ordinary course of keeping himself up to date, but which could be part of the CGK because such materials would have been quickly identified by any person conducting a literature search and review. At the time of that decision, some considered that it was a widening of the CGK, viewing it as no longer requiring the source to be generally reliable and contrary to what Mr. Justice Laddie said in Raych’m Corp’s Patents¹⁷:

The common general knowledge is the technical background of the notional man in the art against which the prior art must be considered. This is not limited to material he has memorised and has at the front of his mind. It includes all that material in the field he is working in which he knows exists, which he would refer to as a matter of course if he cannot remember it and which he understands is generally regarded as sufficiently reliable to use as a foundation for further work or to help understand the pleaded prior art. This does not mean that everything on the shelf which is capable of being referred to without difficulty is common general knowledge nor does it mean that every word in a common text ithers either. In the case of standard textbooks, it is likely that all or most of the main text will be common general knowledge. In many cases common general knowledge will include or be reflected in readily available trade literature which a man in the art would be expected to have at his elbow and regard as basic reliable information.

With this background in mind, the approach to CGK by the current judges when they rely on expert evidence as to exactly what information forms part of the CGK is interesting. In Alcon¹⁸, the claimants disputed the methodology that the defendants’ expert had used to assess that certain information was part of the CGK at the time. However, Meade J. said it is not necessary or realistic to prove exactly by what route the Skilled Person would find or access CGK, and it may well be enough to show that the information is contained in a number of well-known reference-type works. Indeed, the judge was critical of the attempt by Alcon to argue that some texts were too general and known to form part of the CGK. The judge stated that he rejected Alcon’s approach of:

“[S]eeking to rule out general publications merely on the basis that they were general. For example, Rang & Dale’s Pharmacology (2nd Ed, 1991) was a general pharmacology work, and Shields Textbook of Glaucoma (3rd Ed, 1992) was a relatively general medical work about glaucoma, but that does not mean that the skilled addressee would not look to them, in addition to more specialist works, or that they could not form part of the evidence proving common general knowledge. In any event, the former referred to the Coleman classification, though both had a tone consistent with my finding above that the whole field was in an early stage of development.”

Meade J., it seems, takes a pragmatic approach to the knowledge base of experts and CGK, and in general to the instruction of experts. He accepted in Alcon, for example, that as the date in question for considering the CGK was more than 20 years prior, it was reasonable for the expert’s instructing solicitors to provide reference works to the expert for them to opine on.

Other points to note

The final point for this article is to remember that expert opinion should always be supported by facts. In expert reports, every assertion should be supported by citations. If this does not happen, as in Merck¹⁹, the judge is likely to take a very dim view. In that case, Meade J. criticised part of the expert’s report as “speculative ideas” which “should not have [been] advanced without some proper support.

Complementing this, Mellor J. stressed in Fisher that the reasons that an expert comes to a decision is even more important than the decision itself. In that case the expert’s evidence was very thin on each piece of prior art in relation to the reasons for the Skilled Person getting to the idea of one of the claims. The judge was consequently concerned that the expert was reaching the same goal from a very different starting point, and he considered that it was an issue where the expert had not “fully articulated different and independent reasons for each citation.” In consequence, Mellor J. gave less weight to that expert’s conclusions when he came to give his judgment.


When instructing experts, it is important to remember their function – they are there to assist the Court. They can draw on a wide range of common sources to determine CGK, but you must guard against hindsight risk, and instructing solicitors need to think about the order in which they present material to an expert. There is no point in attempting to have them gloss over ambiguous sections in their report, as that would be picked up in cross-examination and the Court needs to know if there is some ambiguity in the CGK or prior art. Nor should the expert provide evidence which is not supported by cited facts. In short, considerable time and preparation should go into the proper instruction of your experts.

1. Philip Morris Products SA v RAI Strategic Holdings Inc [2021] EWHC 537 (Pat) at 186.
2. Teva Pharmaceutical Industries Ltd v Bayer Healthcare LLC [2021] EWHC 2690 (Pat) at 113.
3. Optis Cellular Technology LLC v Apple Retail UK Ltd [2021] EWHC 1739 (Pat).
4. Medimmune Ltd v Novartis Pharmaceuticals UK Ltd [2011] EWHC 1669 (Pat) at 118.
5. Fisher and Paykel Healthcare Ltd v Flexicare Medical Ltd [2020] EWHC 3282 (Pat) at 20.
6. Ibid.
7. Promptu Systems Corp v Sky UK Ltd [2021] EWHC 2021 (Pat) at 16.
8. Ibid at 15(i).
9. Above n. 5 at 22.
10. Above n. 4 at 112.
11. Above n. 1 at 12.
12. Add2 Research and Development Ltd v DSpace Digital Signal Processing and Control Engineering GmBH [2021] EWHC 1630 (Pat) at 17.
13. [2020] EWHC 2636 (Pat) at 30.
14. Alcon Research LLC, Alcon Pharmaceuticals Limited v Actavis Group PTC EHF, Accord-UK Limited, Pharmathen SA, Aspire Pharma Limited [2021] EWHC 1026 (Pat) at 25.
15. Above n. 12 at 13.
16. [2014] EWHC 2873 (Pat)
17. [1998] RPC 31
18. Ibid. at 75.
19. Above n. 13 at 44.