Understanding evidence grades (A–D)
Every OpenBook search result includes an evidence grade. This guide explains what each grade means, how we assign them, and how to use them in everyday clinical decision-making.
The NHMRC Hierarchy of Evidence
OpenBook uses the NHMRC Hierarchy of Evidence — the evidence classification framework used by Australian clinical guidelines, AHPRA, and recognised professional bodies across allied health disciplines. This is the same framework your registration board expects you to reference in CPD submissions.
Grade A — Excellent evidence
Grade A evidence represents the highest level of the NHMRC hierarchy. It comes from systematic reviews that pool results from multiple well-designed randomised controlled trials. A Grade A result means there is consistent, reproducible, peer-reviewed evidence supporting the intervention — the kind of evidence your professional body expects you to follow.
In practice
A search for "constraint-induced movement therapy stroke upper limb" typically returns Grade A evidence because Cochrane has published multiple high-quality systematic reviews on this topic.
Important caveat
Even Grade A evidence must be applied with professional judgement. An intervention may be effective on average across a study population while being inappropriate for a specific patient because of contraindications, comorbidities, goals, or values.
Grade B — Good evidence
Grade B evidence comes from one or more well-designed randomised controlled trials, but without the synthesis and pooling that systematic reviews provide. A single large, well-powered RCT is persuasive evidence — but its findings may not replicate across all populations or settings. Grade B is strong evidence; it supports confident clinical recommendations when applied to appropriate patients.
In practice
Many emerging allied health interventions are supported by Grade B evidence — a well-designed trial may show strong results, but replication studies or formal systematic reviews have not yet been published.
Important caveat
Consider whether the RCT population matches your patient. Publication bias means positive results are more likely to be published than neutral or negative results, so Grade B evidence should be weighed alongside Grade A findings where they exist.
Grade C — Fair evidence
Grade C evidence comes from observational designs that cannot establish causation as definitively as RCTs. These studies describe what happens — who gets better, who gets worse — but cannot fully account for confounding variables. They are valuable in areas where RCTs are impractical (long-term outcomes, rare conditions, real-world practice settings) or where we need population-level data.
In practice
Much of the evidence for dietary interventions in chronic disease management is Grade C — population-based studies have identified associations, but long-term RCTs on diet are logistically difficult to conduct well.
Important caveat
Observational evidence supports practice when it is consistent across multiple studies and settings, but a single cohort study should be treated with more caution. Look for consistency across Grade C sources and any Grade B or A evidence on the same topic.
Grade D — Limited evidence
Grade D evidence is the lowest level of the NHMRC hierarchy. It includes case reports, uncontrolled case series, expert opinion, and consensus statements. This does not mean these sources have no value — for rare conditions or emerging areas of practice, expert consensus may be the best available evidence. But Grade D evidence should not be used as sole justification for an intervention when higher-quality evidence exists.
In practice
Guidance for newly recognised conditions, rare diagnostic categories, or highly individualised interventions often sits at Grade D while the evidence base develops. This does not mean the intervention is wrong — it means the research has not yet been done.
Important caveat
Grade D evidence may be appropriate for practice in the absence of higher-level evidence. Always document your clinical reasoning and consider whether a Grade D recommendation has been endorsed by the relevant professional body or regulatory authority.
How OpenBook assigns grades
OpenBook grades evidence at the level of the individual record, not the aggregate of all sources. Each retrieved record is classified by study type against the NHMRC hierarchy (systematic review → RCT → cohort/observational → case series/expert opinion).
The overall grade displayed in the summary header reflects the highest-quality evidence retrieved — not an average. If a Cochrane review is among the results, the summary shows Grade A regardless of how many lower-level studies are also present.
Within each grade, records are further ranked by recency (more recent publications score higher) and by jurisdiction relevance (Australian sources score higher than equivalent international sources, reflecting the primacy of locally applicable guidance).
Transparency principle
Every citation in an OpenBook summary links to a real record in a verified database. No citations are generated from the AI's training memory — every reference was retrieved during your search. If a record cannot be matched to a verified source, it is discarded before you see it.
Further reading
The full NHMRC Hierarchy of Evidence documentation, including the Level I–V taxonomy underlying the A–D grading system.
NHMRC — How to develop clinical guidelines