Box 151 The quality of evidence rating system reported by Cox et al2

l Evidence obtained from at least one properly designed randomised control trial

Il-i Evidence obtained from well-designed controlled trials without randomisation ll-ii Evidence obtained from well-designed cohort or case-controlled analytical studies, preferably from more than one centre or research group ll-iii Evidence obtained from multiple time series with or without the intervention. Dramatic results in uncontrolled experiments could also be included

III Options of respected authorities based on clinical experience, descriptive studies or reports of expert committees

IV Evidence inadequate owing to problems of methodology (for example sample size, length of comprehensiveness of follow up or conflicts of interest)

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