Expert Evidence and the Duty That Outranks the Client

This is less a set of instructions about specialist opinion evidence in civil disputes than an argument, and it is worth saying so at the start.

Why opinion is treated differently

Witnesses of fact describe what they saw or did, while experts are permitted to offer opinions on technical matters. That permission exists because some issues cannot be understood without specialist knowledge the court does not have. It is an exception to the general position that witnesses give evidence of facts rather than conclusions.

The exception is controlled carefully, because opinion evidence can slide into advocacy dressed up in technical vocabulary that is hard to challenge. The scope of the exception and the controls applied to it differ noticeably between procedural systems and between types of case.

The overriding duty

Most systems require an expert's primary duty to run to the court rather than to whoever instructed them. The requirement is usually stated expressly within the report itself and often repeated in a signed declaration accompanying it. It means an expert should reach the same conclusion regardless of which side approached them first.

For most everyday situations, reports that read as arguments for the instructing party are frequently criticised and may carry little weight. Enforcement of this duty varies between systems, and the practical commercial pressures on regularly instructed experts are well recognised.

Permission and control

Courts in many systems must permit expert evidence before it may be relied upon at a hearing. Permission is typically confined to defined fields of expertise and sometimes to a single expert instructed jointly by both parties. The purpose is to control cost and to prevent disputes becoming contests between competing paid opinions.

As a general position, reports obtained outside those controls may turn out to be inadmissible, which wastes the entire expense of obtaining them. These controls differ substantially, with some systems appointing experts themselves rather than leaving it to parties.

Joint instruction and its consequences

A single expert instructed by both parties reduces cost and narrows the scope for competing opinions. It also means that neither side controls the evidence produced, which parties accustomed to directing their own case find uncomfortable. Questions may usually be put to a jointly instructed expert, subject to rules on how and when.

Where separate experts are permitted, they are often required to meet and identify areas of agreement.

That narrowing exercise frequently resolves more of a technical case than any amount of subsequent argument between the parties does.

What makes an expert opinion useful

Reasoning matters considerably more than the conclusion, since an opinion offered without visible reasoning cannot be tested by anybody. Identifying assumptions is essential, because expert opinions usually rest on factual foundations that are themselves still in dispute.

A good report states the range of professional opinion rather than presenting one view as universal. Acknowledging the limits of the expert's own field is treated as a mark of reliability rather than weakness. Reports failing these standards are regularly given reduced weight even where the underlying expertise is real.

Cost and proportionality

Expert evidence is among the largest costs in technical disputes and can exceed the amount in issue. Courts increasingly weigh whether the likely value of the evidence justifies its expense in the particular case before them. Parties sometimes obtain preliminary opinions before starting proceedings, in order to assess whether a claim is worth pursuing at all.

Such early opinions may attract different protections from those obtained during proceedings, depending on the system. Decisions about instructing experts are properly taken with a qualified lawyer, since the procedural rules governing them are strict and local.

The takeaway

An expert whose opinion changes with the instructing party is worth very little to either of them. General information only, not legal advice.

Understanding a process is not the same as being represented in it.