The supplemental report.
What it is, and is not.

A supplemental report is the expert’s own disclosure, brought current. It is written because the record grew: video produced after a motion to compel, an internal affairs file that arrived late, training and disciplinary records finally located, a deposition taken after the report was served. The subject of a supplement is the enlarged record and what it does to opinions already stated.

A rebuttal report is a different animal with a different trigger. It responds to an opposing expert’s disclosed opinions, on the same record discipline as a primary report, and it exists because of what the other side disclosed rather than because of what discovery produced. Rebuttal work is separately scoped and billed from $5,000. Rebuttal reports and opposing expert review covers that document on its own terms.

Two documents, side by side

• A supplement is triggered by new material in the record; a rebuttal is triggered by an opposing expert’s disclosure.
• A supplement revisits opinions the expert already gave; a rebuttal addresses opinions someone else gave.
• A supplement measures the same questions against a bigger file; a rebuttal measures another expert’s method and sourcing.
• A supplement travels with the original report as one disclosure; a rebuttal stands as its own document.
• Both are written to the same standard: complete record, national standards, opinions sourced to what is actually in the file.

The distinction is not academic. Fed. R. Civ. P. 26 carries a duty to supplement disclosures, and how that duty applies, when it is triggered, and whether a given filing is timely are questions counsel owns. The expert writes what the record supports and states plainly which document is being written; the characterization and the argument belong to retaining counsel.

Supplements live inside the court’s schedule, and that schedule is counsel’s to manage. Discovery cutoffs, expert disclosure dates, dispositive motion deadlines and the trial setting all decide whether a supplement is useful or merely late. The expert can tell counsel how long the work will take; only counsel can say whether the calendar permits it.

Notice matters for the same reason it matters on a primary report. Expert reports require sixty days of advance notice, and reports required in less than sixty days incur a 25% upcharge; a supplement is scoped work on a defined body of new material and is planned the same way. A supplement’s fee is agreed with retaining counsel when its scope is defined, on the same published-schedule discipline as the primary report. Report timelines and the sixty-day rule sets out how the calendar is built.

Late video is the classic trigger. Body-worn and dash camera footage produced after a motion, third-party surveillance located during depositions, and dispatch audio matched to a CAD timeline can all change what the record shows about sequence, distance and warnings. New material of that kind does not always change an opinion, and when it does not, the supplement says so.

Counsel can make supplements cheap by flagging gaps early. If the first report notes what was missing and what remained outstanding, the later production slots into an analysis that already anticipated it, and the supplement is short. If the gap was never identified, the supplement has to rebuild context before it can address the new file. Either way, the opinions stay on police practices, and legal conclusions remain counsel’s.