Table 1.

Level of evidence and type of study

	Therapy /Prevention, Aetiology/Harm	Prognosis	Diagnosis	Differential diagnosis/ symptom prevalence	Economic and decision analysis
		Investigating the effect of patient characteristic on the outcome of disease	Investigating a diagnostic test. Is this diagnostic test accurate?
Level 1	Systematic Review of randomized trials(RT)	Systematic Review of inception cohort studies	Systematic Review of level 1 diagnostic studies	Systematic Review of prospective or classic cohort	Systematic Review of level 1 economic studies
	High quality RT(e.g.:> 80% follow up, narrow confident interval)	Individual cohort study with > 80% follow up, all patient enrolled at the same time	Level 1 diagnostic studies or Validating studies which test the quality of a specific diagnostic test, previously developed, in series of consecutive patients with reference “gold” standard	Prospective or classic cohort studies with good follow up (>80%)	Level 1 studies (analysis based on clinically sensible costs or alternative, values obtained from many studies, and including multiway sensitive analysis
Level 2	Systematic Review of cohort studies	Systematic Review of either historical cohort study or untreated control groups (control arm) in RCTs	Systematic Review of level 2 diagnostic studies	Systematic Review of level 2 studies	Systematic Review of level 2 studies
	Lesser quality RT (e.g.: <80% follow up, wide confident interval, no clear randomization, problems with blinding, etc.)	Historical (retrospective) cohort study or control arm from a RCT	Level 2 diagnostic studies or Exploratory studies which collect information, trawl data to find which factor are significant (e.g.: using regression analysis)	Level 2 studies (retrospective or historical cohort study or with follow up <80%)	Level 2 studies (analysis based on clinically sensible cost or alternative from limited studies, and including multiway sensitivity analysis.
	Individual Cohort study, including matched cohort studies (prospective comparative studies)			Ecological Studies
	Ecological Studies
Level 3	Systematic Review of case–control studies		Systematic Review of level 3 studies	Systematic Review of level 3 studies	Systematic Review of level 3 studies
	Individual case–control study		Level 3 diagnostic studies or studies in non-consecutive patients and without consistently reference “gold” standards	Level 3 studies (non-consecutive cohort or very limited population)	Level 3 studies (analysis based on poor alternative or costs, poor quality estimates of data, but including sensitivity analysis
Level 4	Case-series	Case-series	Case–control study	Case-series	No sensitivity analysis
	Poor quality cohort and case–control studies*	Poor quality cohort and case–control studies*	Poor or non independent reference standard
Level 5	Expert opinion	Expert opinion	Expert opinion	Expert opinion	Expert opinion

A systematic review (SR) is generally better than an individual study. Experimental study (e.g.: good quality RCT) is generally better than any observational study. For observational studies : cohort study is generally better than any case–control study . A case- control study is generally better than any case- series study. * By poor quality cohort study we mean a cohort study that failed to clearly define comparison groups and/or failed to measure exposures and outcomes (preferable blinding) in the same objective way in both expose and non-exposed individuals and/or failed to identify control known confounders and/ or poor follow up. The same for poor quality case–control study except that the patients are identified based on the outcomes in this design ( e.g.: failed replant) called “cases” are compared with those who did not have the outcome (e.g.: had a successful replant) called “controls” and consequently we do not have “exposed and non-exposed” and “longitudinal follow up”. Ecological studies and Economic/decision analysis studies are very uncommon in hand surgery. This chart was adapted from material published by the Centre for Evidence-Based medicine, Oxford, Uk. March 2009.