Expert witnesses may be asked to give evidence in two main situations. Firstly (and most commonly) to give their opinion about whether the standard of care that has been provided is that of a reasonably competent optometrist. This is most likely to be in the form of a written report, but you may also need to give oral evidence to a court. You may feel awkward about giving evidence ‘against’ a colleague, but it is important to remember that the purpose of an expert witness report is to help the court to determine the facts.

The second situation is when an expert is asked to give their opinion about whether a person is able to see well enough to do a particular task, or whether an event that is alleged to have occurred could have occurred, for example would it be possible to ascertain a particular finding from a particular test. This may be in a regulatory or legal case. 

Before agreeing to act as an expert witness you should ensure that you are familiar with the rules of the particular court in the country in which the case is to be heard.¹

When you are acting as an expert witness you have a duty to the court, and not to the person who commissioned you. This means that you must consider the evidence carefully and provide an independent, objective and impartial opinion. If, after you have written your report, you change your view on a relevant matter, or new evidence comes to light that affects your opinion you must make sure that those who have commissioned you, the other party and the judge are made aware of this as soon as possible.

If you do not have the expertise to give your opinion on a particular matter you must make this clear. This would be by either refusing to give your opinion on the matter, or by making it clear that you feel the matter is outside your sphere of expertise.

You should be aware that in exceptional circumstances you may be sued or referred to the GOC for fitness to practise proceedings. This is unlikely to happen and would only happen if you had gross and reckless disregard for your duties.^{2 3}

You must have the necessary expertise in the matters in question and relevant experience in the field. You must also recognise when you are being asked questions that are outside your sphere of expertise. Your expertise should be both theoretical and practical, and in most cases you should be in current clinical practice in the area under consideration, and have been so at the time of the incident in question.

You must know the relevant professional guidance and guidelines, such as that produced by the College and the GOC.

You must be able to communicate clearly and succinctly, both when producing your written report and when giving oral evidence, and be able to explain technical matters in language that is understandable by lay people. 

Training is available for people who undertake expert witness work. This includes how to write a report and how to give oral evidence.

Before writing a report you should make sure that you have researched the area in question. This would include doing a literature search for relevant material. This would include clinical guidelines such as those produced by the College, as well as professional guidance produced by the College, GOC and others. If the matter in question relates to a query relating to an NHS service you should be familiar with the contractual and professional guidance relating to that service. You should describe and assess the worth of any source of the material which you use when forming your opinion. You must not disregard evidence just because it does not fit in with your provisional conclusion.

You must comply with the rules set out by the courts when writing a report¹. Your report must be addressed to the court rather than to the party who is commissioning you and it must be independent, objective and impartial. This means that you should not omit matters that are unfavourable to the person who is instructing you. Your report will be seen by both sides to the case. 

Your report will be read by people who are not experts in your field. The report should be clear and concise, and explain any technical jargon. You should back up your arguments with evidence and make it clear which parts of the report are fact and which are your opinion. You should check that what you have written is correct. It is helpful if paragraphs or lines are numbered so that people can refer to different parts of the report easily. The report should include information about your experience and why you have the expertise to write the report.

You must disclose whether there is information that may adversely affect either your competence or your credibility as an expert⁴. This would include convictions for dishonesty, unresolved complaints or disciplinary hearings against you or previous adverse judicial comment on your evidence.

You should make sure you are familiar with the court procedures relating to the case in question, and the timescales that you must comply with.

If you are required to attend court you will be questioned orally on your report. You should make sure that you know the contents of your report thoroughly, as you may not be able to refer to it when you are in the witness box. You should be prepared for your report to be challenged and you should be able to justify why you said what you did, giving evidence for your assertion where appropriate. Before going to court you should think about what arguments the opposing side may make, and how you will respond to this. That may be by showing that the point made by the opposing side is not relevant, or by explaining why you have the opinion that you do. It is helpful if you can think of examples to back up what you are saying.

Many expert witnesses do not need to attend court. However, you should be prepared to do so. If you are required to give evidence in court you should:

• dress professionally
• arrive on time
• have a confident manner
• be honest, trustworthy, independent, objective and impartial
• not mislead
• answer the question. If you do not understand the question you should ask the questioner to explain what they mean. You must not guess what you think the question is
• stick to the limits of your competence and say if the question asked is beyond your expertise
• keep your answers brief and to the point. You should not give unnecessary information
• where possible do not use technical language. You should be prepared to give examples to explain your point
• keep composed and do not become defensive. You should keep calm if the opposing barrister tries to undermine you, interrupts you or asks you the same question repeatedly
• address the jury if there is one, or the judge or presiding officer if not make everyone aware if you have a conflict of interest or if your opinion changes.

1. Civil procedure rules for: 
   
   England and Wales
   Criminal rules and practice directors
   Information on the Scottish Courts
   Information on courts and tribunals in Northern Ireland
   Protocol for the instruction of experts to give evidence in civil claims (PDF)
   Guidance booklet for expert witnesses June 15 (PDF).pdf
2. Jones v Kaney 30 Mar 2011
3. Phillips & Ors v Symes & Ors [2004] EWHC 2330 (Ch) (20 October 2004)
4. CPS, Chapter 36: Expert Witnesses - Prosecution Disclosure Obligations, Competence and credibility

We would recommend that you also download and read the following: 

• Academy of Medical Royal Colleges (2019), Acting as an expert or professional witness:
  Guidance for healthcare professionals (PDF)
• Crown Office and Procurate Fiscal Office Guidance Booklet for Expert Witnesses (PDF)
• Civil Justice Council - Protocol for the Instruction of Experts to give Evidence in Civil Claims (PDF)