Appendix 3‚ÄÉTypes of study and levels of evidence used to illuminate specific clinical questions

Type of clinical questionEvidence levelBrief definitionTypes of study providing this level of evidence
TreatmentIaA good recent systematic reviewSystematic review of randomised trials
IbA rigorous study designed to answer the questionA rigorous randomised trial comparing T with best alternative
Is therapy T effective?IIOne or more prospective clinical studiesA cohort study or faulty randomised trial
IIIOne or more retrospective clinical studiesA case-control study
IVaFormal expert consensusDelphi study of expert practice
IVbOther informationStudy of pharmacology of T
Aetiology or harmIaA good recent systematic reviewSystematic review of cohort studies
Does A cause disease D?IbA rigorous study designed to answer the questionA large well-designed cohort study
Does drug D cause side effect S?IIOne or more prospective clinical studiesA faulty cohort study
IIIOne or more retrospective clinical studiesA case-control study
IVaFormal expert consensusDelphi study of expert opinion
IVbOther informationStudy of pathophysiology of D
Diagnosis or prognosisIaA good recent systematic reviewSystematic review of blind comparisons of T with gold standard
Is T an accurate test for diagnosis of D?IbA rigorous study designed to answer the questionBlind 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?IIOne or more prospective clinical studiesAnalysis 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?IIIOne or more retrospective clinical studiesRetrospective study of test results or findings in a database of patients with univariate or multivariate analysis
IVaFormal expert consensusDelphi study of expert opinion about T
IVbOther informationStudy of pathophysiology of D
Public health, health policyIaEconomic and policy analysis based on good recent systematic reviewsEconomic 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?IbEconomic and policy analysis based on a rigorous study designed to answer the questionEconomic 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
IIEconomic and policy analysis based on one or more prospective clinical studiesEconomic and policy analysis with modelling and sensitivity analysis using other prospective data in various settings
IIIEconomic and policy analysis based on one or more retrospective clinical studiesEconomic and policy analysis with modelling and sensitivity analysis using retrospective data
IVaFormal expert consensusDelphi study of national expert opinion about P
IVbOther informationLocal opinion about P