Impact measurement & evaluation
Evidence Hierarchy
Definition
An evidence hierarchy is a rank-ordering of kinds of research methods according to their potential to suffer from systematic bias, with the methods least prone to bias placed at the top. Originating in evidence-based medicine — the term was used in a 1979 Canadian Task Force report to grade interventions by quality of evidence — hierarchies conventionally place systematic reviews and meta-analyses of randomised controlled trials highest, followed by individual trials, cohort and case-control studies, case series and, at the base, expert opinion. Dozens of variants exist, including the Oxford CEBM levels and the GRADE approach to rating certainty of evidence.
References
Supports definition and framing.
Supports definition and framing.
Overview
What it means
The hierarchy is a heuristic, not a law: a well-run observational study may outweigh a poor trial, and some sustainability questions (ecosystem dynamics, community outcomes) cannot be randomised at all.
Critics argue rigid hierarchies undervalue qualitative and practice-based evidence, which is why impact-measurement practice has adapted the idea into broader "standards of evidence" that match method strength to the claim being made.
How it is used
Used by evaluators, funders and standards bodies to grade the confidence behind impact claims, to set minimum evidence requirements for funding decisions, and to structure evidence reviews.
Why it matters
Sustainability claims are only as credible as the evidence behind them; a shared hierarchy lets editors, investors and policymakers compare claims on a common footing and spot assertions that outrun their support.
**Note:** Sustainability-sector adaptations (standards of evidence used in UK impact measurement) should be verified against current guidance at publication; the core concept above is the established, citable form.