The highest level of synthesized evidence is 'systems', followed by summaries, synopses of syntheses, syntheses, synopses of single studies, and finally single studies. The original and ongoing intent of classifying evidence in this way is to encourage decision makers to begin their search for evidence at the highest level of the pyramid, and therefore the most synthesized form of evidence, as opposed to beginning their search at the bottom of the pyramid, representing the least synthesized form of evidence. The pyramid can be used to guide decision makers in accessing the highest level of synthesized evidence and only moving to lower levels in the pyramid when no evidence exists at a higher level. It is our experience that public health decision makers generally start their search at the bottom of the pyramid and only come across more highly synthesized evidence by chance.

The term "pre-processed" refers to evidence that has been synthesized and in some cases further summarized into key messages. "Pre-appraised" refers to evidence that has been reviewed for methodological rigour and then in some instances is synthesized, and in other instances is both synthesized and summarized. While there are no internationally accepted criteria as to what constitutes a high quality resource that aims to facilitate access to pre-processed and/or pre-appraised evidence, at a minimum the following guiding principles can be used to assess the quality of the resources described in this paper: a) the process of filtering the evidence so that evidence of the highest quality is chosen for inclusion is transparent and rigorous; b) more than one person is involved in assessing the evidence for methodological quality and in synthesizing and summarizing it; c) the process for assessing, synthesizing, and summarising the evidence is explicit and rigorous; and d) the resource is updated regularly to ensure it remains current. In this paper we provide a critique of only some of the better known resources identified at each level of the pyramid, using these four criteria. However, readers are encouraged to assess for themselves, the quality of the resources identified throughout this paper. The remainder of this paper describes each level of the pyramid and identifies resources at each level that house public health-relevant evidence.

Robeson, P., Dobbins, M., DeCorby, and Tirilis, D. Facilitating access to pre-processed research evidence in public health.  BMC Public Health 2010, 10:95.  doi:10.1186/1471-2458-10-95.

http://www.biomedcentral.com/1471-2458/10/95