The evidence base for this guideline was synthesised in accordance with Scottish Intercollegiate Guidelines Network (SIGN) methodology. A systematic review of the literature was carried out using an explicit search strategy devised by a SIGN Information Officer. Databases searched include Medline, Embase, Healthstar, Cinahl, and the Cochrane Library. The main part of the strategy was based on that used by the Cochrane Library. The year range covered was 1980–2001. Internet searches were carried out on various Web sites including the New Zealand Guidelines Programme, the United Kingdom Health Technology Assessment programme, and the United States National Guideline Clearinghouse. The Medline version of the main search strategies can be found on the SIGN Web site , in the section covering supplementary guideline material. The main searches were supplemented by material identified by individual members of the development group. All selected papers were evaluated by two members of the group using standard SIGN methodological checklists before conclusions were considered as evidence.