An expert witness report is written work, prepared over weeks, checked and re-checked before it is served. Giving oral evidence is none of those things. When a case reaches trial and the judge wants to hear from the experts directly, the format may not be the sequential cross examination most people picture. Under Practice Direction 35 the court can hear both sides’ experts together, in one session, answering the same questions in turn. The formal name is concurrent expert evidence. Almost everyone calls it hot-tubbing.
In short: hot-tubbing lets the court hear opposing experts from like disciplines at the same time rather than one after the other. The judge leads the discussion issue by issue, the parties’ representatives question the experts afterwards, and the judge may then summarise each expert’s position and ask them to confirm or correct it. Preparing for it is not the same as preparing for cross examination, because the expert has to hold and explain a position in front of a peer who disagrees with it.
What concurrent expert evidence actually means
In the traditional format, the claimant’s expert gives evidence, is cross examined and leaves the box. Some time later, perhaps a day later, the defendant’s expert does the same. The judge hears two accounts separated by hours of other evidence and has to reconstruct the disagreement from notes.
In the concurrent format the experts sit together and address the same issue in sequence, while the judge and both legal teams listen to the comparison directly. The disagreement is heard once, in one place, with both experts present to answer it.
Where the procedure came from
The technique developed in the Australian courts and was introduced into the Civil Procedure Rules in England and Wales in 2013, as part of the reforms that followed Lord Justice Jackson’s review of civil litigation costs.
In July 2016 the Civil Justice Council published Concurrent Expert Evidence and Hot-Tubbing in English Litigation Since the Jackson Reforms, a legal and empirical study of how the procedure was being used. It found that judges who had tried it spoke well of it, but that it was being used far less often than expected and in noticeably different ways from court to court. The working group recommended changes to Practice Direction 35 and further guidance, and amendments followed in 2017.
What Practice Direction 35 says
Paragraph 11 of Practice Direction 35 is short, and worth reading in full rather than in summary. Paragraph 11.1 provides that at any stage in the proceedings the court may direct that some or all of the evidence of experts from like disciplines shall be given concurrently. Paragraph 11.2 allows the court to direct that evidence not given concurrently be given in any appropriate manner. Paragraph 11.3 allows the court to set an agenda for the taking of that evidence, issue by issue, or to direct the parties to agree one.
Two points follow from the wording. The first is that this is a case management decision for the judge, not something the parties choose between themselves. The second is that it applies to experts from like disciplines, so a surgeon is heard alongside another surgeon, not alongside an accountant or a care expert.
How the session runs on the day
Paragraph 11.4 sets out three stages, and they happen in that order for each issue on the agenda, or for all issues together.
The judge opens the discussion
The judge initiates the discussion by asking the experts, in turn, for their views on the issues on the agenda. This is the part that surprises experts who have only given evidence in the traditional format. The first question does not come from counsel, and it is not adversarial. It is an invitation to explain a position to a judge who has read both reports and the joint statement and already knows where the disagreement sits.
The representatives ask their questions
Once that process is complete for an issue, the judge invites the parties’ representatives to ask questions of the experts. Cross examination has not disappeared. It happens after the judge has already heard both positions stated side by side, which changes what it can usefully achieve.
The judge summarises and checks
After questioning, the judge may summarise the experts’ different positions on the issue and ask them to confirm or correct that summary. For an expert this is the most important moment in the session. It is the point at which the judge’s understanding is read back, and the last opportunity to say that it is not quite right.
The work that happens before the hearing
By the time a hot-tubbing session starts, most of the ground has already been mapped. Two mechanisms do that work.
The joint statement under rule 35.12
The court may direct the experts to discuss the case, identify the issues and, where possible, reach agreement. Paragraph 9.2 of the Practice Direction is explicit that the purpose of those discussions is not to settle cases but to agree and narrow issues. The resulting joint statement usually becomes the agenda the judge works from.
Two protections sit around it. Under rule 35.12(4) the content of the discussion is not referred to at trial unless the parties agree, and under rule 35.12(5) agreement between the experts does not bind the parties unless they expressly agree to be bound. Paragraph 9.8 adds that where an expert significantly alters an opinion, the joint statement must carry a note explaining the change.
Written questions under rule 35.6
A party may put proportionate written questions to another party’s expert, or to a single joint expert. They must be served within 28 days of the report, may only be put once, and are for clarification of the report unless the court permits otherwise or the parties agree. Answers form part of the expert’s evidence, so they are worth treating with the same care as the report itself.
How the two formats compare
| Feature | Sequential evidence | Concurrent evidence |
|---|---|---|
| Order of evidence | One expert at a time, often on different days | Both experts present, issue by issue |
| Who asks first | The party’s own advocate, then cross examination | The judge, then the representatives |
| Comparison of opinions | Reconstructed by the judge from notes | Heard directly, in the room |
| Agenda | Set by counsel’s line of questioning | Set by the court, or agreed by the parties |
| Typical hearing time | Longer, with repetition across the two sessions | Usually shorter on the contested issues |
Why a court might order it
The usual reasons are time and comprehension. Hearing the disagreement once, with both experts able to respond immediately, tends to take less court time than hearing it twice at a distance. It also tends to sharpen the disagreement, because a position that sounds reasonable in isolation has to survive being stated next to the alternative.
There is a discipline effect as well, and it applies to every expert in the room regardless of who instructed them. An opinion that has drifted towards the case of the party paying for it is harder to maintain in front of a peer from the same field who will be asked, in the next breath, whether they accept it.
Why a court might not
The procedure asks a great deal of the judge, who has to run the discussion and hold the technical detail of two disciplines at once. It suits cases where the issues are well defined and the experts genuinely share a field. Where the disagreement is wide, where the experts’ specialisms only partly overlap, or where credibility rather than reasoning is in issue, the traditional format may serve the court better. The Civil Justice Council study found exactly this kind of variation in practice, which is one reason the procedure is used selectively rather than as a default.
How preparation differs from preparing for cross examination
Preparing for cross examination is largely defensive. The expert anticipates the weakest points in the report and works out how to answer them. Preparing for a concurrent session is closer to preparing to teach. The expert has to be able to state a position clearly enough for a non-clinician to follow it, explain the reasoning that leads there, and say precisely where and why the other expert’s reasoning diverges.
The other difference is that the other expert is listening. In the traditional format an expert can leave a point unaddressed and hope it is not picked up. In a concurrent session someone who understands the field will be asked about it directly, usually within a minute.
Practical preparation for the expert
A few things make the difference between a session that clarifies the issues and one that muddies them.
- Re-read the joint statement immediately beforehand, not the report. It is the document the agenda is built from.
- Know which points were agreed, which were not, and be able to say in one sentence why each remaining disagreement exists.
- Be ready to explain any change of opinion, and check the note required by paragraph 9.8 is there.
- Separate the clinical literature from personal practice, and say which is which when answering.
- Address the judge, not the other expert. The format is a discussion, not a debate to be won.
- Concede what should be conceded, immediately. An expert who defends an indefensible point loses credibility on the points that matter.
None of this changes the duty in rule 35.3, which is owed to the court and overrides any obligation to the instructing party. The format simply makes that duty more visible. Our note on how expert witnesses are regulated in the UK sets out where the duty sits alongside professional obligations.
What it means for instructing solicitors
If concurrent evidence is a possibility, it is worth knowing at the point of instruction whether the expert has given evidence in that format, and whether they can explain a technical position without retreating into jargon. A report that reads well can still be written by someone who becomes vague under questioning from a peer.
It also raises the value of a well-drafted joint statement. Because the agenda usually comes from it, a joint statement that records the disagreement precisely gives the court a better structure to work from than one that lists areas of dispute in general terms. The same discipline that produces a high quality expert witness report produces a usable joint statement.
Frequently Asked Questions
Is hot-tubbing the same as concurrent expert evidence? Yes. Hot-tubbing is the informal name for the procedure set out at paragraph 11 of Practice Direction 35, which the rules call concurrent expert evidence.
Can the parties agree to use it without the court? No. Paragraph 11.1 puts the direction in the hands of the court. The parties can invite the court to make it, and can be directed to agree an agenda for it, but the decision is the judge’s.
Does it replace cross examination? No. Paragraph 11.4 provides for the parties’ representatives to question the experts after the judge has taken their views on an issue. What changes is the order and the context, not whether cross examination happens.
Do experts from different specialisms get heard together? Paragraph 11.1 refers to experts from like disciplines, so the procedure is aimed at experts in the same field. Evidence that is not given concurrently can still be given in any manner the court directs under paragraph 11.2.
What happens if an expert changes their opinion during the session? That is a legitimate outcome and the rules anticipate it. Where the change happens earlier, in discussions between the experts, paragraph 9.8 requires the joint statement to carry a note explaining it. An opinion that moves because the reasoning has moved is doing what the duty to the court requires.
Is a single joint expert ever hot-tubbed? No. The procedure exists to compare opposing opinions from like disciplines. Where the court appoints one expert for both parties there is no opposing opinion to compare, and questions are dealt with under rule 35.6 instead.
Instructing an expert who can give oral evidence
Written work and oral evidence test different things. An expert who understands both is easier to instruct and easier to rely on, whichever side is paying for the report. If you are weighing up a report in a plastic surgery, scarring, hand and upper limb or clinical negligence case, the expert witness services page sets out the scope of instruction, and you can submit an instruction directly.
Related reading
- How to Write a High Quality Expert Witness Report
- Expert Advisor vs Expert Witness, and Why the Difference Matters
- What Is a Single Joint Expert?
This article is general information about civil procedure in England and Wales and is not legal advice or a clinical opinion on any individual case. The Civil Procedure Rules and Practice Direction 35 are amended from time to time, so check the current text before relying on any provision mentioned here.