A change of expert opinion after a report has been disclosed is not, in itself, a sign of a flawed process. It is often the opposite — evidence that the expert has continued to engage honestly with the evidence as the case has developed, rather than defending an initial position out of stubbornness or misplaced loyalty to the instructing party. What determines whether a changed opinion strengthens or damages an expert's credibility is not the fact of the change, but how it is handled.
Legitimate reasons an opinion changes
Several circumstances routinely and legitimately cause a construction or engineering expert to revisit a previously stated conclusion:
- New documents or evidence are disclosed after the initial report was prepared
- Questions or an exchange with the opposing expert during a joint meeting exposes a material inaccuracy or omission in the original analysis
- Evidence the expert originally relied upon turns out, on closer examination, to be unreliable
- Further investigation or testing produces results inconsistent with the earlier conclusion
None of these should be treated as an embarrassment to be minimised or managed quietly. They are the normal consequence of a genuinely independent expert continuing to test their own conclusions as the case evolves — precisely what the overriding duty to the tribunal requires.
The obligation to notify, without delay
The moment a material inaccuracy, omission, or change of opinion becomes apparent, the expert has a positive obligation to notify it — and to do so promptly, not at a moment of the instructing party's choosing. That notification should go to:
- The instructing party
- Other parties to the dispute, typically through their legal representatives
- The tribunal, where appropriate
Delay in making this notification is itself a credibility risk. An expert who becomes aware of a material change but sits on it — whether from reluctance to reopen a settled position, or from a hope the issue will not surface — creates exactly the kind of undisclosed problem that, once it does surface (often through the opposing expert or in cross-examination), does far more damage than a prompt, well-explained amendment would have done.
How the amendment should be documented
The form of the amendment should match its significance:
- Where the change follows a meeting of experts and is relatively contained, a simple, dated addendum or memorandum recording the change and its reasoning is generally sufficient.
- Where the opinion has altered significantly — because of new evidence, because previously relied-upon evidence has become unreliable, or for any other substantial reason — the report itself should be amended, with the amendment including clear reasons for the change, and the instructing party should ensure other relevant parties are informed of the change of opinion as soon as possible.
In either case, the amendment should be transparent about what has changed and why, rather than quietly folding a revised conclusion into a re-issued document without explanation. Tribunals and opposing counsel are entitled to understand the trajectory of an expert's thinking, and an amendment that reads as an attempt to obscure that trajectory tends to attract more scrutiny, not less.
Suggested changes that should be resisted
There is an important distinction between amending a report because the expert's own analysis has genuinely changed, and amending it because the instructing party or legal team has suggested changes the expert does not actually agree with. An expert may properly be invited to amend or expand a report to improve its accuracy, consistency, completeness, or clarity — but must disregard any suggested alteration that does not accord with their true opinion, or that would distort it. Where instructing counsel proposes a change that shifts the substance of the conclusion rather than its expression, the expert's obligation is to decline, and to say so clearly.
This is often where an expert's independence is tested most directly — not in the initial drafting, where the expert controls the process, but in the review cycle, under time pressure, with a client keen to see a stronger conclusion. Holding the line here is precisely what the overriding duty requires, and precisely what protects the expert, and ultimately the client's case, from a far more damaging challenge later.
Why handled well, this builds rather than erodes trust
Tribunals experienced in construction and infrastructure disputes generally understand that complex, document-heavy cases evolve, and that a responsible expert's opinion may evolve with them. What damages an expert is not the change itself, but evidence of concealment, delay, or an unwillingness to acknowledge a position that has become untenable. An expert who amends promptly, explains clearly, and documents the reasoning demonstrates exactly the independence and rigour the role demands — and, in doing so, often does more for their credibility than if the issue had never arisen at all.
Part 11 moves to the next stage of the process: meetings of experts and joint statements, and how these are used to narrow the issues genuinely in dispute before a hearing.