9. The criteria for selecting the evidence should be clearly described

The literature search must balance specificity (the ability to identify papers that closely match the coverage of the question) with sensitivity (the ability to find maximum number of papers of possible relevance). The initial search may therefore produce many papers that have no direct bearing on the key question. Guideline developers can select out these irrelevant papers using explicit inclusion and exclusion criteria that define, for example, the study type or size, geographical location, date, or specific clinical criteria. These should be explicitly described and reasons for excluding evidence should be clearly stated.

A systematic approach should also be taken when assessing the quality of selected evidence. SIGN uses a set of standard checklists to evaluate how well studies of different types (ranging from systemtatic reviews and RCTs to cohort and case control studies) were carried out and reported. SIGN’s approach to the selection and evaluation of the evidence base is described in detail in SIGN 50.

• Search specification
• Example checklist