Carrying out systematic literature reviews: an introduction

Since their inception in the late 1970s, systematic reviews have gained influence in the health professions (Hanley and Cutts, 2013). Systematic reviews and meta-analyses are considered to be the most credible and authoritative sources of evidence available (Cognetti et al, 2015) and are regarded as the pinnacle of evidence in the various ‘hierarchies of evidence’. Reviews published in the Cochrane Library (https://www.cochranelibrary.com) are widely considered to be the ‘gold’ standard. Since Guyatt et al (1995) presented a users' guide to medical literature for the Evidence-Based Medicine Working Group, various hierarchies of evidence have been proposed. Figure 1 illustrates an example.

Systematic reviews can be qualitative or quantitative. One of the criticisms levelled at hierarchies such as these is that qualitative research is often positioned towards or even is at the bottom of the pyramid, thus implying that it is of little evidential value. This may be because of traditional issues concerning the quality of some qualitative work, although it is now widely recognised that both quantitative and qualitative research methodologies have a valuable part to play in answering research questions, which is reflected by the National Institute for Health and Care Excellence (NICE) information concerning methods for developing public health guidance. The NICE (2012) guidance highlights how both qualitative and quantitative study designs can be used to answer different research questions. In a revised version of the hierarchy-of-evidence pyramid, the systematic review is considered as the lens through which the evidence is viewed, rather than being at the top of the pyramid (Murad et al, 2016).