Hierarchy of evidence: a framework for ranking evidence evaluating healthcare interventions.

A number of hierarchies of evidence have been developed to enable different research methods to be ranked according to the validity of their findings. However, most have focused on evaluation of the effectiveness of interventions. When the evaluation of healthcare addresses its appropriateness or feasibility, then existing hierarchies are inadequate. This paper reports the development of a hierarchy for ranking of evidence evaluating healthcare interventions. The aims of this hierarchy are twofold. Firstly, it is to provide a means by which the evidence from a range of methodologically different types of research can be graded. Secondly, it is to provide a logical framework that can be used during the development of systematic review protocols to help determine the study designs which can contribute valid evidence when the evaluation extends beyond effectiveness. The proposed hierarchy was developed based on a review of literature, investigation of existing hierarchies and examination of the strengths and limitations of different research methods. The proposed hierarchy of evidence focuses on three dimensions of the evaluation: effectiveness, appropriateness and feasibility. Research that can contribute valid evidence to each is suggested. To address the varying strengths of different research designs, four levels of evidence are proposed: excellent, good, fair and poor. The strength of the proposed hierarchy is that it acknowledges the valid contribution of evidence generated by a range of different types of research. However, hierarchies only provide a guide to the strength of the available evidence and other issues such as the quality of research also have an important influence.