BRITISH COMMITTEE FOR STANDARDS IN HAEMATOLOGY

CLASSIFICATION OF EVIDENCE LEVELS (AHCPR)

1. a. Evidence obtained from meta-analysis of randomised controlled trials.
   b. Evidence obtained from at least one randomised controlled trial
    
2. a. Evidence obtained from at least one well-designed controlled study without randomisation.
   b. Evidence obtained from at least one other type of well-designed quasi-experimental study. *
    
3. Evidence obtained from well-designed non-experimental descriptive studies, such as comparative studies, correlation studies and case studies.
    
4. Evidence obtained from expert committee reports or opinions and/or clinical experiences of respected authorities.

STRENGTH OF RECOMMENDATION:

     A.  Requires at least one randomised controlled trial as part of a body of literature of overall
          good quality and consistency addressing specific recommendation. 
          (Evidence levels Ia, Ib).

    B.  Requires the availability of well conducted clinical studies but no randomised clinical
          trials on the topic of recommendation.  
         (Evidence levels IIa, IIb, III).

    C.  Requires evidence obtained from expert committee reports or opinions and/or clinical
          experiences of respected authorities. Indicates an absence of directly applicable clinical
          studies of good quality. 
          (Evidence level IV).