Rigour of development

Guidelines derived from a consensus of expert opinion without prior systematic literature surveys are liable to bias in their conclusions and recommendations and may not reflect current medical knowledge. Guidelines should therefore be based on a systematic review of the evidence. This means that the literature should be identified according to an explicit search strategy; selected according to defined inclusion and exclusion criteria; and evaluated against consistent methodological standards.

Forming recommendations from this validated evidence base will necessarily involve elements of subjective judgement as to the consistency, clinical relevance and external validity of the evidence. The factors considered in formulating recommendations should be clearly described and consistently applied.

Finally, guideline recommendations should be linked to the evidence on which they are based. The grading system used by SIGN assigns the highest level of evidence to meta-analysis of randomised controlled trials, and the lowest to expert committee reports or consensus statements. This categorisation reflects the likely validity of the study results, taking into account issues such as predictive power and susceptibility to bias.