Insights
When Expert Evidence Goes Wrong – The 5 Common Mistakes
Expert evidence can be decisive in complex litigation, particularly in cases involving financial markets, regulation, accounting, tax, technology or another specialist field. The court may depend heavily on experts to understand the subject matter and assess the competing arguments. Technical knowledge alone, however, does not necessarily make expert evidence reliable. An expert may fully understand the subject but still fail to assist the court.
This series considers five common ways in which expert evidence can go wrong.
Part 1 examines the expert who becomes an advocate and gradually loses the necessary distance from the party who instructed them.
Part 2 considers the expert who helps construct the case. It will examine how an expert’s early explanations and opinions can become absorbed into counsel’s understanding and eventually form part of the pleadings that the expert is later asked to support.
Part 3 looks at the expert who answers the wrong question, including the tendency to stray into matters of law, intention, credibility or factual determination.
Part 4 considers the report that cannot be tested because the evidence, assumptions and reasoning behind its conclusions are not made clear.
Part 5 addresses the expert who is unprepared for challenge, including experts’ meetings, joint statements and cross examination.
The common theme is that expert evidence does not fail just because the technical analysis is wrong. It can also fail because the expert has misunderstood the role, lost independence or failed to communicate the reasoning in a way that properly assists the court.
Part 1: The Expert Who Becomes an Advocate
An expert’s primary duty is not to the party who instructs them or pays their fees. It is to assist the court by providing independent evidence within their area of expertise. The principle is simple. The expert must maintain that independence throughout what may be a long and complex engagement without becoming an advocate for the instructing party. The loss of independence may not result from a deliberate decision, it is more likely to occur gradually. The expert will naturally spend considerable time with one legal team, reading the evidence through the issues raised in that party’s case. They will discuss weaknesses in the opposing position and help counsel understand technical matters. There is nothing inherently improper about this, as effective expert work requires close cooperation with lawyers. The danger arises when cooperation develops into identification with and support for the client’s position. It is easy for the expert to begin thinking in terms of “our case” and therefore to regard criticism of the client’s arguments as criticism of their own work. This can cause the expert’s role to shift from setting out a properly justified opinion to attempting to defend the client’s position.
At that point, the expert risks becoming an advocate.
Independence does not mean neutrality
An independent expert does not have to be, and is certainly not expected to be, neutral on every issue. An expert may reach a firm conclusion that strongly supports one party and substantially undermines the other. An opinion that assists the instructing party is not necessarily partisan. The important point is that the opinion must be the expert’s own conclusion, based on an objective assessment of the evidence and the proper application of their expertise. If new evidence emerges, the expert must be willing to reconsider the opinion.
An expert who is no longer capable of changing their view is no longer approaching the evidence independently.
How an expert can creep into advocacy
The following are examples of how an expert can gradually move from independence into advocacy:
- The report adopts the language of the pleadings without independently testing whether it accurately describes the technical position.
- The evidence is not scrutinised objectively. The instructing party’s evidence may be examined in detail while contrary material is dismissed too quickly or omitted.
- Assumptions favourable to the client are accepted without proper analysis.
- Conduct capable of more than one explanation is presented as proving the interpretation preferred by the legal team.
- Qualifications disappear as drafts progress and conclusions are expressed with greater certainty than the evidence permits.
- The expert strays into matters outside their expertise because the answers support the client’s case.
Each step may appear minor but, taken together, they can turn an independent opinion into specialist advocacy.
The drafting process
An expert report will usually be discussed with the instructing lawyers. They may identify unclear passages, question whether a conclusion is adequately supported or point out that an issue has not been addressed. This is legitimate and often improves the report. The process becomes problematic when its purpose shifts from improving accuracy and clarity to making the opinion more useful to the client. Lawyers may help refine the wording and structure of the report, but the opinions must remain those of the expert. A qualified conclusion should not be made absolute merely because stronger wording would better support the pleaded case.
Unhelpful evidence should not be removed simply because it complicates the argument, nor should reasonable concessions be deleted because opposing counsel may rely upon them.
Objective analysis and testing the case
One of the clearest signs of advocacy is the selective treatment of evidence. An expert may examine documents supporting a favourable view in detail, while applying a less demanding standard to material pointing in the opposite direction. Inconsistencies in the client’s evidence may be explained away, while similar inconsistencies in the opposing case are treated as decisive. The expert may identify every weakness in the other expert’s analysis without acknowledging comparable limitations in their own. This approach may make the report appear stronger at first, but it makes the evidence more vulnerable. Opposing counsel will readily identify omitted or underdeveloped material. The expert may then have to explain why apparently relevant evidence received little or no attention.
An independent expert should therefore address the strongest evidence against their conclusions, not merely the evidence that supports them. That does not mean giving every document or explanation equal weight. It means explaining why contrary evidence does not alter the opinion or why an alternative interpretation is less persuasive.
The expert must also test the technical foundations of the client’s case rather than simply identifying material that supports an initial view. A useful question is, “Would I express the same opinion if I had been instructed by the other side?”
The expert should also consider:
- Have facts and assumptions been clearly separated?
- Are there assumptions that still require proof?
- Has the strongest evidence against the conclusions been properly considered?
- Have reasonable alternative explanations been examined?
- Have the same analytical standards been applied to both parties?
- Is any part of my opinion outside my expertise?
- Would the court understand why one interpretation has been accepted and another rejected?
These questions should be considered throughout the engagement, not only when the report is ready to be signed.
The danger of overstatement
Some experts appear to believe that an absolute and strongly expressed conclusion is more valuable to the client but, in practice, excessive certainty often weakens the evidence.
Commercial events are rarely reconstructed from perfect information. Documents may be incomplete, witness accounts may conflict and more than one explanation may be commercially plausible. An expert should not disregard those uncertainties simply because they are inconvenient. Where the evidence supports a firm conclusion, it should be stated clearly. Where uncertainty remains, it should be explained. Where alternatives are possible, they should be addressed and reasons given for preferring one.
Careful qualification is not weakness. It demonstrates that the expert understands the limits of the evidence and has not confused opinion with certainty.
The role of the instructing lawyers
The expert is personally responsible for maintaining independence, but the conduct of the instructing lawyers also matters. A good legal team does not ask an expert merely to validate the pleaded case, but tests the expert’s reasoning, asks difficult questions and identifies weaknesses before the report is served. An experienced legal team will also recognise the value of an expert who is prepared to say that a particular proposition cannot properly be supported.
The strongest relationship between counsel and expert is not one in which the expert agrees with every aspect of the client’s case. It is one in which both understand that the expert’s credibility depends on the independence of their judgment.
Concessions and changes of opinion
An advocate is expected to advance the client’s case, whereas an expert must be prepared to make concessions where the evidence requires them. A properly made concession can strengthen credibility. It demonstrates that the expert is considering each issue on its merits rather than defending a fixed position.
The same applies where an opinion changes. New documents may emerge, an assumption may prove wrong, or another analysis may cause the expert to reconsider the original view. Changing an opinion for good reason is not a failure. The greater danger is resisting change because the original opinion has already become important to the client’s case.
An independent expert should explain what has changed, why it has changed and what effect the change has on the conclusions.
The value of independence
One of the most important roles of an independent expert is to tell the client what they may not want to hear. Identifying an unsustainable argument may prevent it from becoming central to the case. Recognising uncertainty may allow further evidence to be obtained. Making an appropriate concession may increase the court’s confidence in the expert’s more important conclusions. Once the court concludes that an expert is acting as an advocate, technical knowledge alone may not restore credibility. The most persuasive expert is rarely the person who supports every aspect of the instructing party’s case. It is usually the person whose opinions are demonstrably independent, carefully reasoned and capable of surviving proper scrutiny.
The expert’s value lies not in loyalty to the client, but in the reliability of the judgment.
srb@objectivus.com
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