The Background section is based on neither a systematic review nor comprehensive literature searches. Some of the Evidence in smaller type may now be out of date. Where possible, information included in the previous Manual based on on-going reviews has been replaced by more recent material. Evidence is graded A (derived from randomised controlled trials - RCTs), B (observational studies) and C (professional consensus). These are broad categories and the quality of evidence within each category varies widely. Thus it should not be assumed that RCT evidence (grade A) is always more reliable than evidence from observational studies (grade B)
