Substantiation is the evidence a business holds to prove the objective claims in its marketing. Under rule 3.7 of the CAP Code, you must have that documentary evidence before an ad is published, not gather it after someone complains.
How substantiation works
The rule applies to claims a reader is likely to take as factual and that can be checked: “the UK’s No.1”, “London’s leading”, “the fastest”, “rated 5 stars”, “cuts energy bills by a third”, “over 500 happy customers”. Obviously subjective statements, such as “a lovely place to stay”, are puffery and do not need proof, but the line is narrower than most people think. The Advertising Standards Authority tends to read “leading” and “best” as objective claims about market position or performance.
When a complaint arrives, the ASA asks the advertiser for evidence. The evidence has to match the claim as an ordinary reader would understand it. A survey of fifty existing customers cannot support “the UK’s favourite”; a ranking for one search term on one day cannot support “No.1 on Google”; a case where one client’s leads doubled cannot support an implied promise that every client’s will. If the evidence does not cover the claim, the ad is ruled misleading, and the ruling is published with your business name.
The same principle runs through consumer law and the CMA’s guidance. A comparative advertising claim needs evidence about the competitor as well as yourself. A testimonial must be genuine and documented, and cannot carry a claim you could not make directly. Environmental claims under the Green Claims Code need the same discipline.
Why it matters
Search and social ads reward bold, short claims, and limited character counts push advertisers towards superlatives. “London’s best plumber” or “No.1 SEO agency” is easy to type into a headline and hard to defend. Competitors know this, and they are a common source of ASA complaints.
It also matters for trust. Readers have learned to discount unsupported superlatives, while a specific, checkable statement, such as “Gas Safe registered, with fixed prices published on our website”, persuades better. This site applies the same rule to itself: a result, ranking or client number appears only where the evidence exists and the client has agreed to it being published.
Common mistakes
- Calling the business “leading”, “No.1” or “the best” with no data on market share, sales or independent rankings behind it.
- Quoting a review score without the date, source and number of reviews, or after the score has dropped.
- Promising outcomes, such as results guaranteed within thirty days, that depend on things outside your control.
- Using an old survey or award long after its date, without saying when it was.
- Collecting evidence only after a complaint, which fails the rule even if the claim turns out to be true.
- Letting an agency write claims nobody in the business has checked.
How to act on it
Go through your ads, landing pages, homepage and social bios and highlight every claim that could be checked. For each, either file the evidence, with its source and date, in one shared folder, or rewrite the claim so it says only what you can prove. Replace superlatives with specifics: what you do, for whom, where, and what a customer can verify.
Make the check part of sign-off, so no new ad goes live with an unsupported claim. When I write ads for Google search campaigns, every headline that states a fact is matched to evidence the client has supplied before the campaign launches.