Evidence Half-Life

Evidence half-life is the interval over which a research finding still supports a decision before market, audience, or platform shifts erode it. A pricing survey from eight months ago may still hold; a competitive positioning study from the same quarter often does not, because AI answer engines now rewrite category narratives faster than annual research cycles. For agencies, the framework converts research from a one-off project into a scheduled asset: each deliverable carries a stated expiry, and renewal conversations are timed to when the evidence, not the contract, runs out. HubSpot's October 2026 AEO guidance is a working example, since brand descriptions inside ChatGPT, Claude, and Gemini drift as third-party sources change, which means a visibility baseline captured in Q3 needs re-querying before it can anchor a Q1 content scope. Pair self-reported data with observational signals so the expiry date reflects behavior, not just stated intent.