Expert Witness Journal Issue 68 August 2026 - Flipbook - Page 32
times before a single con昀椀dent conclusion can be
reached. Sometimes a single viable hypothesis is just
not possible.
at this point. An expert who has genuinely tested
all plausible hypotheses from both perspectives,
and explained why the evidence favours one over
the others, has satis昀椀ed PD35 3.2(6). They are
also prepared for cross-examination in a way that
an expert who has not cannot be. The opposing
argument has been tested from the inside. There are
no surprises.
Two requirements: both shared by
engineering and the law
Engineering discipline and civil procedure each
impose two related but distinct requirements on
expert investigations. It is worth understanding
them separately, because they tend to fail for
di昀昀erent reasons — and, as the case law shows, they
can be violated independently of each other.
Rigorous recording: documenting
the investigation
The second requirement is less dramatic but
equally fundamental. A scienti昀椀c investigation is
reproducible only if it is properly recorded. Every
piece of evidence examined, every document
reviewed, every instruction received and every
assumption made must be identi昀椀ed, attributed and
disclosed, so that the basis of the opinion can be
understood and tested.
The 昀椀rst is analytical impartiality: The obligation to
test every plausible hypothesis against the evidence
from both directions, rather than selecting evidence
that supports a preferred conclusion. The second is
rigorous recording: The obligation to document
what evidence was examined, what information
was relied upon, and the basis for every opinion
expressed. Both are required by good scienti昀椀c
practice. Both are required by the procedural rules.
Both can be independently violated. Both failures
lead to the same outcome — an opinion the court
cannot trust.
CPR 35.10(3) requires the expert’s report to state
the substance of all material instructions, whether
written or oral. PD35 paragraph 3.2(3) requires
identi昀椀cation of the documents and materials relied
upon. These are not bureaucratic formalities — they
are the procedural expression of the same principle
that underlies scienti昀椀c reproducibility: an opinion
that cannot be traced to its evidential foundation
cannot be properly evaluated.
Analytical impartiality: testing every
hypothesis from both sides
An expert who holds a preferred conclusion will
examine evidence selectively. They will identify
features that support their hypothesis and give
less weight to those that complicate it. They will
articulate one narrative clearly and leave competing
narratives unstated. The infamous ‘con昀椀rmation
bias’.
In practice this means maintaining a clear audit trail
throughout the investigation: what components
were examined, in what condition, by whom; what
information was provided by the instructing party
and when; what tests were conducted and under
whose supervision; and critically, what information
reached the expert through channels other than
the formal instruction process. The discipline of
recording is inseparable from the discipline of
independence — because undisclosed information
reaching one expert but not the other corrupts the
level playing 昀椀eld on which expert evidence depends.
The di昀케culty with this approach is well understood
by experienced litigators. Opposing counsel,
advised by an expert who has examined the same
material, will know precisely which features were
left unstated. The one-sided analysis is technically
fragile in a predictable way: it has already identi昀椀ed
the pressure points that cross-examination will 昀椀nd.
The human factors that undermine both
Both requirements are undermined by recognisable
human behaviour patterns that have nothing to
do with de昀椀ciencies in the legal framework or the
engineering methodology.
The rigorous approach is to treat each plausible
hypothesis as a proposition to be tested from both
the claimant’s and the defendant’s perspective:
what evidence supports this failure mechanism,
and what evidence is inconsistent with it? Does
the inconsistency falsify the hypothesis or merely
reduce its probability? If two hypotheses survive that
testing, what further evidence would distinguish
between them, and is it available?
Analytical impartiality fails through con昀椀rmation
bias — the tendency to give greater weight to
evidence that supports a pre-existing view — and
through what might be termed advocacy drift: the
gradual alignment of an expert’s framing with the
narrative of the instructing party. These are not
usually deliberate distortions. They are patterns that
emerge naturally from the adversarial environment
in which expert evidence is produced, and they
require conscious and sustained counter-e昀昀ort to
resist.
This is what competent root cause analysis requires.
It is also what Practice Direction 35, paragraph
3.2(6) requires: a mandatory summary of the range
of opinion, including legitimate contrary views, with
reasons for the opinion expressed. The procedural
obligation and the analytical obligation converge
EXPERT WITNESS JOURNAL
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AUGUST 2026
