One of the greatest misconceptions surrounding expert evidence is that consistency is always a virtue. It is often assumed that an expert who never departs from their original opinion is demonstrating confidence, conviction and independence, whilst an expert who modifies their conclusions somehow exposes weakness in their analysis. In reality, my experience has often been precisely the opposite. Over the years I have acted as an Expert Witness, Single Joint Expert, adjudicator and mediator, represented parties relying upon expert evidence and, more recently, reviewed the work of other experts in professional negligence claims. Through those different roles I have reached a simple conclusion. The expert who concerns me most is rarely the one who changes their opinion. More often, it is the expert whose opinion never changes at all.
The role of an Expert Witness is fundamentally different from that of an advocate. An advocate advances a client’s case as persuasively as possible within the confines of their professional obligations. An Expert Witness, however, owes an overriding duty to the Court or Tribunal which overrides any obligation owed to the party instructing them or paying their fees. That principle lies at the heart of CPR Part 35, the accompanying Practice Direction, the Civil Justice Council Guidance for the Instruction of Experts in Civil Claims and the recently published RICS Surveyors Acting as Expert Witnesses (5th Edition). The expert’s responsibility is not to defend the opinion they first expressed. It is to assist the decision-maker by providing an objective opinion based upon the evidence available at the time that opinion is given.
That duty necessarily requires intellectual honesty. It also requires intellectual flexibility. Expert evidence is rarely prepared in a static environment. Construction disputes evolve throughout litigation, arbitration and adjudication. Additional disclosure takes place, witness statements are exchanged, site inspections identify matters previously unknown, testing is undertaken and assumptions made at the outset of an instruction may subsequently prove to be incomplete or incorrect. It would therefore be surprising if an expert’s thinking never developed as the evidential landscape evolved around them. A willingness to reconsider an opinion in light of new evidence is not a sign of uncertainty; it is an indication that the expert continues to discharge the very duty for which they were appointed.
This principle is far from new. In National Justice Compania Naviera SA v Prudential Assurance Co Ltd (The Ikarian Reefer) [1993] 2 Lloyd’s Rep 68, Cresswell J made clear that where, after the exchange of reports, an expert changes their view having considered the opposing expert’s report or for any other reason, that change of opinion should be communicated without delay. The significance of that observation is sometimes overlooked. The Court was not criticising experts who revise their opinions. On the contrary, it was recognising that genuinely independent experts should remain open to changing their conclusions if the evidence justifies doing so. An expert who refuses to revisit their reasoning despite a materially different evidential picture may inadvertently demonstrate a greater concern with defending an earlier opinion than fulfilling their overriding duty to the Tribunal.
The same philosophy runs through the Civil Procedure Rules. CPR Part 35 is not designed to encourage experts to become entrenched in opposing positions. Rather, it promotes cooperation between experts, the narrowing of issues and the identification of those matters that genuinely require judicial determination. The provisions relating to discussions between experts and the preparation of joint statements would serve little purpose if experts regarded any movement from their original position as an admission of weakness. Those discussions are intended to test assumptions, explore areas of agreement and disagreement and, where appropriate, refine or modify opinions in light of reasoned professional dialogue. In many respects, an expert who participates constructively in that process is demonstrating precisely the independence that the Rules seek to promote.
Robert Home and John Mullen make a similar observation in The Expert Witness in Construction. They identify flexibility as one of the defining characteristics of a competent Expert Witness, explaining that an expert should be prepared to consider additional facts and adjust their views where appropriate, warning that an expert who refuses to alter their opinion regardless of new information risks crossing the line into advocacy. They further observe that experts should remain willing to reconsider their conclusions after receiving additional information or after considering the opinions of other experts and, where appropriate, should amend their views at the earliest opportunity. Those observations are entirely consistent with both CPR Part 35 and the principles established in The Ikarian Reefer.
In my own practice, some of the most productive stages of litigation occur during meetings between experts. Properly conducted, those meetings are not exercises in negotiation or compromise, nor should they be viewed as contests to determine which expert can defend every point made within their original report. Their purpose is to allow two independent professionals to scrutinise one another’s reasoning, test assumptions against the available evidence and identify whether areas of disagreement genuinely remain. Sometimes those discussions reinforce an expert’s original conclusions. Equally, they may expose an assumption that can no longer be sustained or identify evidence that materially affects an opinion. Neither outcome should be regarded as failure. Indeed, they demonstrate that the process is functioning exactly as intended.
There is, however, an important distinction to be drawn. An expert who changes their opinion because additional evidence requires them to do so is demonstrating professional independence. An expert who changes their opinion simply to accommodate the wishes of those instructing them is doing precisely the opposite. Equally, an expert who refuses to acknowledge compelling evidence because it undermines a previously expressed conclusion is not displaying confidence or strength. They may simply have become emotionally invested in defending their own report. That is a risk to which every expert is susceptible, regardless of experience or reputation.
In my previous article I considered the dangers of confirmation bias and the gradual effect that prolonged exposure to one party’s narrative can have upon even the most conscientious expert. This article represents the natural continuation of that discussion. One of the clearest indicators that confirmation bias may have taken hold is an unwillingness to reconsider an opinion when the evidence changes. Once an expert becomes more concerned with defending yesterday’s conclusions than reaching today’s correct answer, the line between independent expert evidence and advocacy begins to blur.
Having considered expert reports from a number of different perspectives, including as an Expert Witness, adjudicator and reviewer of expert evidence in professional negligence claims, I have come to appreciate that the most persuasive experts are seldom those who never move from their original position. Rather, they are those who demonstrate intellectual curiosity, analytical rigour and the professional confidence to say, where appropriate, that further evidence has caused them to refine or even change their opinion. Far from undermining their credibility, that approach frequently enhances it because it demonstrates that their loyalty remains firmly with the evidence rather than with a conclusion reached at an earlier stage of the proceedings.
Ultimately, independence is not demonstrated by never changing your opinion. It is demonstrated by being prepared to change it when the evidence genuinely requires you to do so. The expert whose opinion develops through careful investigation, thoughtful analysis and reasoned reflection is usually doing precisely what the Court, Tribunal or adjudicator expects. It is the expert whose opinion remains entirely unmoved despite an evolving evidential picture that causes me to pause. After many years acting in a variety of dispute resolution roles, I have found that the statement, “Having considered the additional evidence, I have changed my opinion,” rarely gives me cause for concern. More often than not, it reflects an expert who understands their overriding duty. It is the expert who insists that nothing has altered their thinking, regardless of everything that has emerged during the proceedings, who is usually the one I worry about most.