Treatment | Ia | A good recent systematic review | Systematic review of randomised trials |
| Ib | A rigorous study designed to answer the question | A rigorous randomised trial comparing T with best alternative |
Is therapy T effective? | II | One or more prospective clinical studies | A cohort study or faulty randomised trial |
| III | One or more retrospective clinical studies | A case-control study |
| IVa | Formal expert consensus | Delphi study of expert practice |
| IVb | Other information | Study of pharmacology of T |
Aetiology or harm | Ia | A good recent systematic review | Systematic review of cohort studies |
Does A cause disease D? | Ib | A rigorous study designed to answer the question | A large well-designed cohort study |
Does drug D cause side effect S? | II | One or more prospective clinical studies | A faulty cohort study |
| III | One or more retrospective clinical studies | A case-control study |
| IVa | Formal expert consensus | Delphi study of expert opinion |
| IVb | Other information | Study of pathophysiology of D |
Diagnosis or prognosis | Ia | A good recent systematic review | Systematic review of blind comparisons of T with gold standard |
Is T an accurate test for diagnosis of D? | Ib | A rigorous study designed to answer the question | Blind prospective comparison of T, F or M with gold standard for D or E (eg, response to specific therapy) with multivariate analysis |
Is finding F an accurate predictor of event E? | II | One or more prospective clinical studies | Analysis of prospective test results in patients enrolled in a randomised clinical trial of therapy for varying stages of D. Prospective validation study with univariate analysis |
Does severity measure M accurately predict event E? | III | One or more retrospective clinical studies | Retrospective study of test results or findings in a database of patients with univariate or multivariate analysis |
| IVa | Formal expert consensus | Delphi study of expert opinion about T |
| IVb | Other information | Study of pathophysiology of D |
Public health, health policy | Ia | Economic and policy analysis based on good recent systematic reviews | Economic and policy analysis with modelling and sensitivity analysis using data from systematic reviews of effectiveness and of cost studies in the same routine clinical settings |
Is policy P cost effective in the NHS? | Ib | Economic and policy analysis based on a rigorous study designed to answer the question | Economic and policy analysis with modelling and sensitivity analysis using data from a randomised clinical trial of effectiveness and a cost study in the same routine clinical setting |
| II | Economic and policy analysis based on one or more prospective clinical studies | Economic and policy analysis with modelling and sensitivity analysis using other prospective data in various settings |
| III | Economic and policy analysis based on one or more retrospective clinical studies | Economic and policy analysis with modelling and sensitivity analysis using retrospective data |
| IVa | Formal expert consensus | Delphi study of national expert opinion about P |
| IVb | Other information | Local opinion about P |