Concurrent Expert Evidence and "Hot-tubbing" in England & Wales

United Kingdom


On 1 August 2016, the Civil Justice Council’s Civil Litigation Review Working Group published its final report on the use of concurrent expert evidence in civil trials.

Concurrent expert evidence was formally introduced as part of the wide-ranging Jackson reforms, on the basis that it has the potential to save time and costs, improve the quality of expert evidence and assist the court in understanding complex issues. The primary purpose of the Working Group’s project was to consider how the practice has fared since it was formally implemented in April 2013.

As acknowledged by the Working Group, despite the potential advantages of expert evidence being given concurrently, there is a perception in the legal community that the process is rarely used in litigation. The Working Group sought to understand, by way of a survey of the judiciary, legal practitioners and expert witnesses, why (or why not) the process was being used, how the process was being deployed and whether or not its aims were being achieved.

The outcome of the project is a range of recommendations, including proposed amendments to Practice Direction 35.11 (concurrent expert evidence), a suggested new Guidance Note for Judges and Practitioners and new Information Note for Expert Witnesses, and some further suggestions to improve familiarity with the process.

Concurrent expert evidence – a variety of forms

Concurrent expert evidence (commonly referred to as “hot-tubbing”) is a discretionary procedure by which the court may order expert evidence to be heard. Practice Direction (PD) 35.11 sets out the procedure to be followed in those circumstances and, although the procedure may be modified at the judge’s discretion, as noted by the Working Group, the drafting of PD 35.11 seems to suggest that there is only one procedure. In actuality, there are multiple procedures that may be adopted and these are explored in greater detail in the report.

In fact, both before and after its formal implementation in April 2013, concurrent expert evidence has been employed in a variety of forms in English courtrooms, including as follows:

- sequential, back-to-back evidence: widely used in professional negligence claims, experts are sworn in together but give evidence sequentially (“back-to-back”), with a traditional examination and cross-examination following each expert’s evidence on each issue;
- judge-led joint expert examination (“hot-tubbing”): on each issue, generally the judge will question each expert in turn, perhaps inviting the experts to comment on each other’s responses, before allowing a limited form cross-examination if appropriate and then moving on to the next issue;
- hybrid forms of hot-tubbing: there are a number of potential variations, whereby the court may permit greater interaction between experts in the hot-tub and/or allow counsel to play a greater role in directing questioning; and
- a form of teach-in: the parties’ experts are sworn in concurrently and each participates in a carefully controlled tutorial (a “teach-in”) at a general level on the issues in dispute, to aid the understanding of the court.

Taking account of the various forms that concurrent expert evidence may take, the Working Group recommends that the Civil Procedure Rules (CPR) be amended to recognise that hot-tubbing is just one form of concurrent expert evidence and that particular care is taken with terminology to ensure that the full range of options are explored and ultimately used to the benefit of all parties involved in litigation.

When is hot-tubbing appropriate?

Although the report makes clear that this technique has much to offer parties, and the courts, the report contains a caveat that concurrent expert evidence will not be appropriate in every case. In order to assess whether or not this is the case, the Working Group’s proposed Guidance Note for Judges and Practitioners sets out matters to which a judge or tribunal should have regard. The list, which is not exhaustive, includes the following issues:

- judicial preparation time: given that hot-tubbing is a judge-led process, a judge should consider whether they have sufficient familiarity with the subject matter and sufficient time to prepare before making the order;
- experts’ personalities: a judge may feel that the personality of one expert is such that they may be at a significant advantage or disadvantage in giving evidence in the hot-tub, relative to the other experts involved;
- numerical imbalance: in cases where one side’s experts are numerically outnumbered by the other, this may be a factor that cautions against ordering hot-tubbing; and
- issues of credibility: any issues between the parties on the narrow issue of expert credibility (that is, allegations of bias or lack of independence) are likely to be unsuitable for hearing in the hot-tub format (however, the report notes that this should not preclude a hybrid procedure, where some evidence is given in the hot-tub and other issues heard in the traditional manner).

The key message is that hot-tubbing will not be appropriate in all cases but hybrid procedures may be suitable where issues that are unsuitable can be isolated and heard separately in the traditional manner.

Other issues

The report considers some other important issues that may be of concern to parties considering the use of concurrent expert evidence.

Procedural Fairness

Whilst it is clearly important that the hot-tubbing process, when deployed, achieves some or all of its objectives, it is more important that the process does not give rise to issues of procedural fairness. This was of particular concern to the Working Group and in their report they considered certain features of the process that might lead to complaints of inequality – these include opening statements by experts, the layout of the courtroom, testing the expert’s evidence and the order and type of questions which the judge asks the experts. In relation to the latter, for example, of the expert witnesses surveyed, almost 40% reported that they were not asked the same questions as their opposing expert (44% of legal representatives surveyed reported witnessing the same trait).

In response to these concerns, the Working Group prepared a Guidance Note for Judges and Practitioners, identifying the issues to be aware of when considering the logistics of concurrent expert evidence.

“Loss of control”

The report notes that a perceived “loss of control” of the witness was a reason frequently given in responses to the Working Group’s survey as to why hot-tubbing was not more popular. These perceptions are not surprising where, in fact, the role of counsel is partially displaced by the judge who leads the process and thus (potentially) the questioning of the expert witnesses. What is arguably more relevant is the effect this has on the process – despite perceptions, those who were surveyed most frequently remarked that the process was simply different from traditional examination and cross-examination (rather than worse).


Practice Direction 35.11 currently provides that full cross-examination of the expert witnesses following the hot-tubbing process is neither necessary nor appropriate. However the survey responses provided a very different picture of what is happening in English courtrooms. Those surveyed from the judiciary reported that in 33.3% of cases, each counsel conducted a full cross-examination of the other side’s expert witness and in only 16.7% of cases did each counsel decide that no further examination was necessary.

Reflections on the success of hot-tubbing

As noted above, concurrent expert evidence was formally introduced as part of the wide-ranging Jackson reforms in April 2013 on the basis that it has the potential to save time and costs, improve the quality of expert evidence and assist the court in understanding complex issues. The report reflects on the success of hot-tubbing, making the following observations as to whether it achieves these objectives:

- time and costs: although time may be saved at trial, it was noted that the increase in preparation time meant that overall, the net effect might be negligible. As to costs, of the legal practitioners surveyed, only 28% thought that hot-tubbing led to a costs-saving to the parties;
- quality: the overwhelming majority of judicial respondents and legal practitioners considered that the quality of expert evidence was improved by hot-tubbing, though the experts surveyed were more ambivalent, with 30% stating that no improvements at all were gained; and
- aid to understanding: the overwhelming majority of judicial respondents and legal practitioners considered that the court’s understanding of the issues was improved by hot-tubbing, and the majority of experts surveyed were also supportive in this respect.

Next steps

Following the publication of the report, it will now be up to the Civil Procedure Rule Committee (CPRC) to decide whether or not to amend the CPR in accordance with the Working Group’s recommendations. It is anticipated that the CPRC will find the time to do this after the summer holiday period, and that the other recommendations will follow on from there.


The Working Group has carried out a thorough investigation of a procedure which has significant potential to save time at trial, to provide better quality evidence and assist the court’s understanding of complex issues. The report highlights the issues that parties should be aware of when considering the process, and provides valuable insight into the manner in which the process has been deployed to date. For anybody considering whether or not concurrent expert evidence would be appropriate to their case, the report is an essential read.

For a copy of the report please click here

Guy Pendell, CMS Partner and Solicitor Advocate, is a member of the Civil Litigation Review Working Group.