There is increasing interest in clinical guidelines in Britain. With this interest has come increasing awareness of the methodological issues in the development of valid guidelines.1 2 3 Practice guidelines are considered valid if “when followed, they lead to the health gains and costs predicted for them.”1 When appropriately disseminated and implemented, valid guidelines can lead to changes in clinical practice and improvements in patient outcome.4 5 6 7 Conversely, the dissemination and implementation of invalid guidelines may lead to wasteful use of resources on ineffective interventions or, worse, deterioration in patients' health.

Validity has been related to three principal factors in guideline development—namely, the composition of the guideline development panel and its processes; the identification and synthesis of evidence; and the method of guideline construction.5 Though these factors have been discussed at theoretical5 and more practical levels,8 there have been few attempts to put them into practice in Britain. In this series of three papers we describe the methods used to develop evidence based guidelines for the primary care management of two common chronic conditions—namely, asthma in adults and stable angina—and summary versions of the two guidelines that resulted.9 10