Expert evidence in delay and quantum disputes

The duty

Primary law In England and Wales, CPR Part 35 and Practice Direction 35 impose an overriding duty to the court that takes precedence over any duty to the instructing party, and prescribe what a report must contain. In US federal courts, Rule 702 requires reliable principles and methods reliably applied. Australian evidence statutes and court expert codes impose their own reasoning requirements.

What the report has to separate

1. Instructions: what was asked, and by whom.
2. Facts: what the expert was told, and what they verified.
3. Assumptions: what was taken as true without verification, flagged as such.
4. Method: what was done, why it was chosen and what was rejected.
5. Opinion: the conclusion, and its limits.

Blurring facts and assumptions is the most common structural failure. An assumption that migrates into the narrative as a fact is the thing the other side will find, and once found it colours everything else in the report.

Reproducibility

A reader should be able to rebuild the analysis from the source files. That means the native programmes, the model as run, the logic edits, the sensitivity cases and an audit log of what changed. An opinion that cannot be reproduced is an assertion, however well presented.

How opinions fail under cross-examination

• The expert decided a legal question, usually responsibility, and coloured the analysis to match.
• A method was chosen without the alternatives being considered on the record.
• The as-built logic turned out to be the expert’s own, undisclosed.
• Contrary evidence in the record was not addressed.
• The range of reasonable opinion was not stated, so any concession looked like a collapse.
• Conclusions rested on experience rather than on exposed reasoning.

Joint statements

The purpose is to narrow the issues, not to win them. A joint statement that records precisely where two experts disagree and why is more useful to a tribunal than one that reads as two positions restated. Agreeing facts early costs nothing and buys credibility on the points that matter.

Where AI-assisted work fits

A tool in the chain does not change the duty. It becomes part of what must be explained: what it did, what it proposed, what the expert accepted and why. Systems designed as proposal plus evidence plus named reviewer survive that question. Systems that produce conclusions do not.