I Evidence obtained from at least one properly designed RCT Il-i Evidence obtained from well-designed control trials without randomisation ll-ii Evidence obtained from well-designed cohort or case-control 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 Opinions 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 or comprehensiveness of follow up or conflicts in evidence)
derived from the US Task Force on Preventative Care Guidelines.37,38 We have only presented data from the best quality studies; for example, data from comparative studies is presented preferentially over data from case series when both exist for a given intervention. The grading system is given in Box 28.1.
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