Expert evidence – hot tubbing

One of the most interesting proposals contained in Lord Justice Jackson’s fundamental review of the rules and principles governing the costs of civil litigation, published on 14 January 2010, was the proposal to pilot the use of the Australian practice of concurrent expert evidence, colloquially known as witness conferencing or “hot tubbing”, an idea that is beginning to play an increasing role in International Arbitration as well. The basic idea is that experts from the same discipline are sworn in at the same time and the tribunal then chairs a discussion between the experts. The format will obviously differ from hearing to hearing, but one way to proceed is to use any joint expert statement (recording the matters upon which the experts disagree) as an agenda. Counsel are able to put questions to the experts and the experts themselves can question each other. The suggestion made is that hot tubbing provides a cost-effective and efficient means of cutting through what the expert evidence really means and what the experts themselves genuinely believe. Is that right or is this just a fad, whose popularity has spread simply because of a name, which is both catchy and a boon for commentators and humourists alike?

As noted above at page 5, Lord Justice Jacksons’s proposal has been adopted as part of the Revised TCC Guide. The Guide notes that often there is a stage prior to this when each of the experts gives evidence conventionally and is cross-examined on general principles and other matters. The experts are then invited to give evidence concurrently on particular issues. Procedures vary but, for instance, a party may ask its expert to explain his or her view on an issue, then ask the other party’s expert for his or her view on that issue and then return to that party’s expert for a comment on that view.

Alternatively, or in addition, questions may be asked by the judge or the experts themselves may ask each other questions. According to the Guide, the process is often most useful where there are a large number of items to be dealt with and the procedure allows the court to have the evidence on each item dealt with on the same occasion rather than having the evidence divided sometimes by a number of days, as different experts are cross-examined.

This theory is confirmed by those with experience of the process in Australia. Gary Edmond, a professor at the University of New South Wales School of Law,1 has said of hot tubbing:

“The openings of these sessions tend to be more informal than examination-in-chief (that is, direct) and cross-examination, which are associated with conventional adversarial proceedings…at least part of their testimony, experts are freed from the constraints of formally responding to lawyers’ questions. During concurrent-evidence sessions, expert witnesses are usually presented with an opportunity to make extended statements, comment on the evidence of the other experts, and are sometimes encouraged to ask each other questions and even test opposing opinions.”

His conclusion is interesting, noting that whilst the practice of hot-tubbing:

“is not a panacea for partisanship, adversarial bias, or the difficulties created by expert disagreement and decision making in the face of uncertainty ... [it does have] the potential to improve communication and comprehension in the courtroom.”

The basic idea behind hot-tubbing seems to be that as the process is more informal or relaxed, there will be the opportunity for the experts to engage in constructive discussion. Ideally, this will encourage and/or enable them to reach a higher degree of consensus. As the experts are being questioned together there is the opportunity for detailed discussion on particular issues or even the opportunity for immediate rebuttal, something that is impossible with traditional cross-examination.

Certainly, one can see that by having the experts answer questions together, it will be easy to identify areas where there is true disagreement and it will be easier for an expert to clarify or correct errors or inaccuracies in his evidence, partly because of the less-adversarial nature of the process, perhaps partly because of immediate peer pressure. An expert knows he is likely to be picked up on any such points by a colleague sitting next to him. In theory, it may also be of benefit where there are complex and complicated issues. Cross-examination, with its emphasis on short responses, is not always the best forum to do this. Hot-tubbing would seem to allow for a more expansive approach to explanations of more tricky areas.

However, this is also one of the potential risks of the system. In a more relaxed environment, will your expert perhaps concede more easily ground he would hold over rigorous cross-examination? Whilst you would hope that this is a problem that would never arise, partly because if there were concessions that needed to be made, a good expert would already have indicated to you what they were, and partly because a party has the right to expect that an expert is properly and thoroughly prepared for every circumstance. Hot-tubbing is no different to cross-examination in that regard. However, it is fair to say that as with any new process, those with experience will, at least initially, have an advantage which they may be able to press home.

Obviously, too, the Tribunal must be fully prepared. Unlike cross-examination, it is the Tribunal that takes the lead. This does raise one potential problem. Does this impose a restraint on a party’s right and ability to lead the case it wants and to put the questions it wants? Whilst hot tubbing will enable parties to see which issues are of greatest concern to the Tribunal at perhaps an earlier stage in proceedings than might otherwise be the case, there seems to be a risk that a party might not have the opportunity to make all the points they need to raise or bring forward all the documents that they would like the Tribunal and experts to see.

It is important, therefore, that the advocates take a positive and active role in the process and are not passive bystanders allowing the Tribunal to take complete charge. Perhaps the best way is for the Tribunal to start the process off, and then allow input from the advocates as the discussion progresses.

Also, when it comes to questions of credibility and independence, the informal relaxed atmosphere (and this of course can only be in relative terms) of the hot tub does not really seem to be the ideal forum to examine such issues. Most parties would want these to be brought out in formal cross-examination. Perhaps, therefore, there is scope for a two-stage process involving both formal cross-examination and the joint session.

Finally, there is the question of costs, the real driver of the Jackson Report. However, here one must question whether there is any real benefit. Hot-tubbing takes place during the hearing. Therefore all the costs of the hearing will have already been incurred. Further, an expert will have to prepare just as thoroughly for the joint session as he would for cross-examination. Accordingly, it is difficult to see where there will be any saving in costs at all.


Hot-tubbing is already a feature of International Arbitration, and whilst strictly whether it becomes a feature of litigation (or perhaps adjudication) in the UK might well depend on the success (or otherwise) of the pilot scheme. It does seem likely that it is going to become more prevalent in the UK. Therefore it cannot be ignored.

So what steps can you take to ensure that you, your expert and legal team are properly prepared? The first is to understand the process as best you can. Ensure that a clear set of ground rules has been agreed so that everyone understands how the process is going to run. You should then ensure that everyone understands those rules.

There is nothing unusual about the second step: make sure that everyone, from expert to advocate, is properly prepared and thoroughly understands what the key issues are and what issues in particular you want to raise before the Tribunal. Hot tubbing does not provide any easy answer, as the expert who has not done his homework will find to his cost, whether hot tubbing with his peers or being subject to cross-examination in the traditional way.

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  • 1. Merton and the Hot Tub: Scientific Conventions and Expert Evidence in Australian Civil Procedure (2009).

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