Essential Do’s and Don’ts for Expert Witnesses

5 Nov '19

The expert is now closely regulated by the rules. Provided that they conduct themselves in an honest, competent and careful professional manner they are unlikely to transgress the rules. However, experience has shown that there are pitfalls, pitfalls which may have serious consequences.

By Alec Samuels

The Rules and Practice Direction

The expert must not omit always to have with them a copy of the Rules CPR 35 and the Practice Direction and the Guidance for the Instruction of Experts in Civil Claims 2014, to study them, to understand them, to refer to them, and to follow them.  A hard copy in the file and the ability immediately to access them on the computer are part of the legal tools of his trade.  


The expert must give all their qualifications.  Where they are more of a practical person than a university or theoretical person they should not omit full details of their practical experience leading to their practical knowledge and their justification for claiming expert status.  They may not be formally well educated, but they have worked on these machines, or they have worked as a senior manager in the building trade, or as a police officer they have worked in drugs or gang culture or road accidents for many years  Myers v The Queen [2015] UKPC 40, [2016] AC 314. 

Inadequately instructed

From time to time the expert may feel that their instructions are inadequate.  They should not just “make the best of a bad job”, they should seek more information from those instructing then, or mark their report “provisional” and explain why, or in extreme situations they may apply to the Judge for guidance.  They should not go out searching for facts, that is not their role; they should ask for them to be supplied.  They must be provided with the material necessary for their work.  

Range of practices and opinions

Often there is a possible range of practices and opinions, not simply a question of right or wrong.  The expert should indicate the range, in summary form, and in choosing one opinion give the reasons for their own opinion in preference to the others.  They should always make full disclosure, and be open and transparent.  The opinion should be clear and confident, but depending upon the circumstances does not necessarily have to be dogmatic, rejecting any possible alternative opinion.  There may be a balance involved, “pros and cons” of various practices, and in their opinion the balance falls in favour of the view advanced.  Using mathematical figures may be dangerous, but a positive approach is to be recommended.  In court the nature of the balance will almost certainly be raised.  

Respond to the other side

Depending upon the complication and length of the negotiations and pre-trial process, when a new issue is raised, and especially when the other side ask for more information, the expert should promptly respond, through the lawyers, so that the issue does not emerge belatedly, inadequately considered, for the first time, during the trial.  This will not please the judge.  For example, consideration whether the hospital trust or the local authority should pay for a disabled child, and whether at home or in an institution, was left until a very late stage.

How long?

The expert will want to put into the report all the relevant and material matters, as they feel that they must not miss anything.  The judge will want a short snappy report going to the heart of the substantive issues, setting out analysis and opinion, not all the factual history, available elsewhere.  So the expert may find themselves in a bit of a dilemma.  Therefore the best procedure may be to compile a full detailed report with a short executive summary and also including a summary of the real issues which appear to be in dispute.  The well-prepared expert will anticipate the technical or expert points likely to arise at trial, and prepare accordingly.  

Managing the costs in the case is for the lawyers, but the expert cannot ignore thinking of the costs involved in their involvement and participation and the desirability of brevity, expedition and keeping costs under control, subject to the needs of justice.  

The gold standard

The expert must not fall short of the gold standard:  Independent.  Impartial.  Objective.  Unbiased.  Never an advocate.  Duty to the court.

The expert is not and cannot be expected to be a lawyer.  The latest authoritative Supreme Court view of the duty of the expert is to be found in Kennedy v Cordia [2016] UKSC 6, [2016] 1 WLR 597, paras 38-71 – evidence of a consulting engineer in a health and safety case.  

Conflict of interest

There must be no conflict of interest with a party if independence, impartiality and objectivity are to be protected from challenge.  The relationship with the client must be strictly at arm’s length.  Ideally there should have been no connection whatsoever between the client and the expert before this case.  Potentially fatal situations: They are of the same family or group of friends or social circle or club, or they have worked together in business or as colleagues in the same institution.  For example, they are customer and supplier, architect and client, doctor and patient, tutor and student – one way and another they have had dealings in the past.  Preferably they should never have had anything to do with each other.  Perception may be as powerful as reality.  The strict legal requirement can pose a dilemma or difficulty in a small community or speciality, where everybody knows everybody.  The expert must be an “outsider”. If there is a possibility of a “relationship” it must be disclosed – not accidentally revealed or discovered during the trial.  

Basis for the opinion

Unsupported assertion, mere anecdote, is no good.  The instructions should be exhibited.  Then the methodology, the research, the tests, the measurements, the calculations, the plans, the surveys, the photos, the literature, the references, whatever, must be set out, and copies of material given to the other side if there are access problems.  So far as possible everything should be checked and verified.  Material which is deemed irrelevant or insignificant or otherwise not relied upon should not just be ignored but a brief explanation given.  Everything must be relevant, nothing irrelevant.  What the expert must most definitely not do is to suppress or distort any material which is relevant but which “does not fit” their theory and opinion.  The report must not fall short of being a reliable opinion compiled on the basis of all the material supplied.  

In context

Full references should be given, for ease of access for everybody.  Page 10 in the text book may look to be very supportive of the opinion advanced by the expert.  But the other expert will look at the book too, and read pages 9 and 11, and page 10 may then take on a very different aspect and in context bear a rather different interpretation.  

Limit of expertise

The expert must not stray outside their expertise.  They should not pontificate upon the law, they are no expert on the law.  Counsel may seek to lure them into unfamiliar territory, to sidetrack them, to get them to display a lack of knowledge, to reveal inadequacies, to make errors, the “Meadow” syndrome.  For the expert proudly and arrogantly to suggest that they are some sort of polymath shows a serious weakness and will be ruthlessly exposed.  The “wandering” expert can undermine the credibility and force of even the evidence they give where they really are an expert; they will suffer a sort of pervading corrosion.  The advocates and the judge should ensure that the evidence of the expert is confined to their expertise and is relevant and admissible Meadow v General Medical Council [2006] EWCA Civ 1390, [2007] QB 462, para 206.

Following the receipt and reading of their instructions the expert may take the view that the nature, complexity and importance of the case necessarily demand or require expert evidence on a matter going beyond their own expertise.  Therefore they should as soon as possible draw this matter to the attention of those instructing them.

These days the pressure is on for expedition and cost saving, and a party seeking leave to call an expert has to show that the expert is “necessary”, or at the very least reasonably required.  Where a party seeks leave to call two experts, alleging that two different specialities are involved, the pre-trial case management judge has a tendency to give leave only for one report, saying that that expert can cover the entire ground.  Thus may the one expert be tempted or drawn beyond their expertise.  They should strongly resist, and insist on keeping within their own professional expertise, and no more, and not venture beyond.  

Change opinion

Assuming that the original report is careful and professional, the expert must not be afraid of changing or modifying their opinion.  Flexibility is better than obstinacy.  New information may become available, the factual basis may change, the expert report on the other side may be persuasive.  Modifying an opinion may well lead to a narrower and clearer identification of the real issues in the case.  The judge will respect the thoughtful, reflective expert who is constantly searching for the “right” or the “best” answer.

If the expert does change their mind they must at once inform those instructing them and the other parties involved in the case.  

Asked to change their opinion

Having seen the report, those instructing the expert occasionally ask them to delete certain passages or change them, as they are seen as damaging to the case for the client.  The expert should absolutely refuse, and if necessary withdraw from the case.  Of course, rectifying an obvious mistake drawn to their attention is perfectly proper, and an explanatory note should be added.  Where new professional information becomes available, or the report of the opposing expert contains persuasive contrary information, then changing the report is very desirable, not to say imperative. But this is for good professional reasons, not because of improper pressure.

The lay response  

The sensitive expert recognises that they are always dealing with laymen, by definition to a greater or lesser extent ignorant of the expertise of the expert.  Therefore they must not forget that effective communication is the vital skill, intelligibly and clearly explaining the expertise so that the layman may understand.  The lawyers are likely to be reasonably well informed; the judge will usually have a quick intelligent understanding; in a criminal case some of the jurors will usually be persons of very limited education and understanding, though often shrewd in judging honesty and genuineness.

The expert should not fail to appreciate that their evidence may be received with a degree of cynicism, as with the psychologist giving evidence of human behaviour because we laymen all think that we know and understand human behaviour and motivation and emotions, or with the psychiatrist saying that the client is ill and not responsible for his actions because of addiction to drugs or alcohol or gambling or sexual predation. Some expert evidence, for example mathematic or chemistry or medicine, may seem to the layman to be simply inherently unintelligible and therefore make no impact.  Expert evidence is, however, essential in order to establish diminished responsibility in answer to a charge of murder, as opposed to voluntary intoxication and alcohol addiction and the consequences.  

The “facts”

In a number of cases the expert in a sense has to rely upon what the client tells him, e.g. a road or industrial accident or hospital mishap.  However, the expert should not just accept what they are told: they should state what they have been told and go on to probe and to appraise the situation.  They are not there to advance the case for the client but independently, impartially and objectively to give their opinion in order to assist the judge.  The expert may disagree with the case advanced by the client and the legal advisers: so be it.  Quite often there is a dispute between claimant and defendant on the facts: Who was there, who did what, who said what.  The expert was not there, so they themselves cannot say what happened.  All they can say is that on the basis of the facts advanced by A the proper professional practice in their opinion would have been so and so, and on the alternative basis of the facts advanced by B the proper professional practice in their opinion would have been the alternative so and so.  It is absolutely no part of the role of the expert to say that they prefer one version to the other, to say that one is more reliable than the other, that one should be believed and the other not.  That is to usurp the role of the judge.  

Answer the questions

In court the expert simply answers the questions put to them, not the questions not put to them, or which they thinks should be put to them.  The conduct of the case rests with others.  The answer answers the questions, it does not go into a long exposition, the court does not need to know how clever they are or thinks they are.  At some stage the judge is likely to put the important questions, if they seem to have been overlooked; and usually at the end of giving their evidence the expert will be asked if there is anything really relevant not raised that they wish to say; or they can specifically ask the judge if they may give an opinion on some matter which they feel has been overlooked or distorted, and the judge will decide.  The good expert always keeps calm. Counsel may try to rile, annoy, irritate or provoke them, by belittling their standing, challenging their competence, even worse.  Counsel may seek to draw them into an unseemly argument.  The questions may seem irrelevant and ill-informed and even stupid.  The expert must not be provoked into frustration or anger.  Remember that counsel is in their own customary environment, they are on home ground, they will get the better of the argument if it comes to a “slanging match”.  So the expert is a model of restraint.  The judge will recognise what is going on.

Help the judge not the client  

The expert should always remember that they are there to help the judge, not the client.  They are uninfluenced by the exigencies of litigation.  They are there to put a well-founded expert opinion before the judge.  They will explain and defend that opinion if challenged.  That opinion may or may not be accepted by the judge.  No matter, the opinion was soundly based and well presented. 


The expert must be disinterested in the result.  They are not there to help the “client” win, they are there to assist the judge in reaching the correct decision on the facts and the law.  They are not the decision-maker.  They may be professionally interested in the technical issues, and what the other expert says, they may indeed learn something from the case, but they do not care who wins or loses.  They may not have the full picture.  They are not a lawyer, they are no expert on the law, they are not the judge.  They may speak, with authority, on the best practice in the particular situation.  They cannot say who is and who is not legally liable.  They do not ever act as an advocate.  They do not speak “for” their client.  They do not say that their client is “a good person”, honest and reliable.  They merely give their expert opinion on the facts and the questions put to them.  


The expert owes to the court and to those instructing them the duty of care of a standard properly to be expected of an expert in that situation.  Thus the engineer should have appreciated the risk of fire in the engine; the surveyor should have discovered the risk of subsidence; the doctor should have diagnosed the heart condition  Jones v Kaney [2011] UKSC 13, [2011] 2 AC 398.

The most precious thing for the expert is their professional reputation, for integrity and skill.  If they are incompetent or careless a variety of adverse consequences may flow.  Their evidence may be declared to be inadmissible.  Their evidence may be rejected.  The judge may publicly criticise them.  The party instructing them may unnecessarily lose the case.  Costs may be awarded against them, and those instructing them, and the party concerned.  They may be reported to their professional body.  They may never receive instructions again.  They may be sued in negligence by those instructing them, the party who called them, even the opposing party.  Their reputation will be damaged or destroyed.  

Alec Samuels is a retired Barrister and member of the EWI.


Want to learn more?

EWI’s popular report writing  and advanced report writing courses provide essential training to ensure your reports are compliant. We also have new dates for our Confidence in the Courtroom training.