“I cannot answer that” can sound uncomfortable in the witness box. Used properly, however, it is not an admission of weakness. It is evidence that the expert understands the boundaries of the role.
When the question falls outside the expert’s field
The clearest reason to decline an answer is that the issue lies outside the expert’s expertise.
Practice Direction 35 requires experts to make clear when a question or issue falls outside their expertise. The GMC similarly states that medical professionals should give opinions only on matters within their competence or relevant knowledge. Where a question falls outside that area, they should explain the position and decline to answer or comment. This sounds straightforward until disciplines begin to overlap.
An orthopaedic surgeon may properly comment on a musculoskeletal injury and its physical consequences. That does not necessarily qualify them to diagnose post-traumatic stress disorder. A psychiatrist may assess the claimant’s mental state but may not be equipped to give a detailed opinion on the biomechanics of a spinal injury. A general practitioner can have considerable experience of managing a condition without possessing the specialist knowledge required to assess complex surgery.
The question is not whether the expert knows something about the subject. Most experienced clinicians know something about neighbouring fields. The issue is whether their training, practice and experience qualify them to provide the opinion the court has requested.
Experts should identify this problem when they receive the letter of instruction, not after the report has been served. The GMC requires an expert to consider in each case whether they have the knowledge, training and experience needed to assist on the issues. If the instruction extends beyond that scope, the solicitor should be told promptly. Accepting an instruction and quietly avoiding the difficult questions is not a solution.
When the evidence is insufficient.
An expert may be well qualified to answer a question but lack the information needed to do so.
Practice Direction 35 expressly requires experts to say when they cannot reach a definite opinion because the available information is insufficient. The GMC also requires an expert to make clear when there is not enough information to reach a conclusion or when an opinion must be qualified because the evidence conflicts. This situation arises frequently in medico-legal work, earlier medical records may be missing. Imaging may not be available for review. The claimant may give an account that differs materially from the contemporaneous notes. The expert may be asked to assess a condition without examination, or to distinguish accident-related symptoms from a poorly documented pre-existing disorder.
The correct response is not always a complete refusal to express an opinion. Sometimes the expert can provide a conditional view: “On the history given, this would be consistent with the accident, but I cannot reach a concluded opinion without the earlier records.”
That is different from presenting an assumption as an established fact. A useful report identifies what is missing, explains why it matters and states whether the deficiency prevents an opinion entirely or merely reduces confidence in it. The court can then decide what weight to place on the evidence or whether further information should be obtained.
When the question belongs to the court.
Experts are often asked questions that are dressed in medical language but require a legal or factual decision. Whether a claimant is honest is not a medical diagnosis. Whether a witness is reliable is a matter for the court. Whether the defendant is legally liable is not decided by the expert, even where the expert gives evidence on breach of duty or causation.
An expert may identify inconsistencies, explain whether reported symptoms are clinically plausible and discuss whether examination findings support the alleged disability. The expert should not convert those observations into a declaration that the claimant is lying unless the professional discipline and evidence genuinely justify such an opinion. The same restraint applies to disputed facts, and an expert should not choose which witness is telling the truth and then build the report upon that choice.
Where different factual accounts would produce different opinions, the proper course is to explain the alternative conclusions.
Current judicial guidance in the coronial jurisdiction follows the same broad principle: facts are for the court or jury to determine, and an expert should provide alternative opinions where the conclusion would change under different factual scenarios.
“I cannot decide which account is correct” may therefore be the only proper answer.
When science does not permit certainty.
Some questions cannot be answered as precisely as the lawyers would prefer. The expert may be asked for the exact date on which a condition developed, the precise percentage contribution of several causes or a definite prognosis where the recognised outcome is variable. A neatly expressed figure can look helpful, but it may have no proper clinical foundation.
The expert should distinguish between reasonable professional judgement and invented precision. A range may be supportable where a single figure is not. A balance-of-probabilities, opinion may be possible even though absolute certainty is not. In other cases, the evidence may support several explanations without allowing the expert to select one reliably. Practice Direction 35 requires experts to consider all material facts, including those that detract from their opinion. The GMC requires the expert to identify the facts or assumptions underlying the view, address any range of professional opinion and explain how the preferred conclusion was reached.
Uncertainty should be explained, not concealed behind technical language.
When the question is unclear.
An expert should not answer a question they do not understand, poorly drafted instructions may combine several issues, contain an incorrect factual assumption or ask the expert to apply an unfamiliar legal test. The GMC requires experts to ensure that they understand the questions and to seek clarification where instructions are unclear.
The answer may therefore be: “I cannot answer that question as presently framed.”
The expert should then identify the ambiguity: Is the question asking about diagnosis, causation, prognosis or treatment? Does “caused by the accident” mean the sole cause, a material contribution or simply that symptoms followed the event? Which factual account is the expert being asked to assume?
Clarification is preferable to a polished answer to the wrong question.
“I cannot answer” should not become a device for avoiding an inconvenient point. Experts have a duty to answer properly put written questions, and failure to answer may affect whether the instructing party can rely upon the evidence or recover the expert’s fees. A proper refusal therefore needs a reason.
The expert should say whether the issue is outside their expertise, whether evidence is missing, whether the question is reserved for the court or whether no reliable professional conclusion can be reached. They should also say whether another specialist, further records or a differently framed question would assist.
The same discipline is required in oral evidence. There is no shame in pausing, asking for the question to be repeated or correcting an answer that has gone too far. The greater risk lies in defending an unsustainable opinion simply because it has already been expressed. The court does not require an expert who can answer everything. It requires an expert who knows which answers can safely be trusted.
Sometimes the most helpful evidence is: “I cannot answer that, and this is why.”
