Evidence Hierarchy (Evidence-Based Medicine)

The explicit ranking of evidence types used in evidence-based medicine (EBM) and — by extension — evidence-based policy and evidence-based management. Higher-ranked evidence trumps lower-ranked, regardless of the causal question.

The canonical pyramid (from highest to lowest)

Meta-analyses / systematic reviews of RCTs
Randomised Controlled Trials (RCTs) — the gold standard
Comparative / observation studies (cohort, case-control)
Statistical correlation studies
Experimental methods (bench science)
Mechanistic reasoning / expert opinion

The course presents this hierarchy in sources/2023-anjum-rocca-phi403-lecture-01-metascience (figure on the back of the handout) and again in sources/2023-anjum-rocca-phi403-lecture-12-explanatory-power-of-mechanisms.

The philosophical bias

The hierarchy's ranking encodes a philosophical commitment to a Humean / difference-making theory of causation: correlation-based evidence is treated as more secure than mechanistic evidence. This biases intervention against causal claims that are hard to operationalise via RCTs (complex interventions, rare effects, individual-level questions).

Andersen, Anjum & Rocca 2019 ("Philosophical bias is the one bias that science cannot avoid", eLife) is the course's reference paper on this — see concepts/philosophical-bias.

The course's critique

Mechanisms hold more explanatory power than regularities (sources/2023-anjum-rocca-phi403-lecture-12-explanatory-power-of-mechanisms).
RCTs systematically exclude causally relevant information (concepts/rct-limitations).
The Russo-Williamson thesis advocates pairing statistical and mechanistic evidence rather than ranking one above the other.
Methodological pluralism (concepts/methodological-pluralism) rejects a fixed hierarchy in favour of context-sensitive evidence weighting.

concepts/rct-limitations · concepts/methodological-pluralism · concepts/causal-pluralism · concepts/mechanisms-in-science · concepts/philosophical-bias