Table 2.

Preferred Hierarchy of Articles Used in the Effectiveness Review^*

Study/Publication Type	Study/Publication Relates to Efficacy	Study/Publication Relates to Effectiveness
Meta-analyses†	+
Systematic reviews‡	+ (especially when part of a meta-analysis)
Evidence-based guidelines		+
Individual randomized clinical trials	+ (unless it is an effectiveness trial)
Observational studies		+
Case–control studies		+
Clinical practice guidelines based on consensus or opinion, rather than on evidence		+

^*Note: Exceptions to the hierarchy may occur, depending on the methodology used in each study. Studies or reviews critically based on evidence are given more weight.

^†Particularly those included in the Cochrane library.

^‡Particularly those performed by authoritative organizations such as the Agency for Healthcare Research and Quality, U.S. Preventive Services Task Force, Evidence-based Practice Centers, National Institutes of Health, and Centers, for Disease Control.