Classifying scientific evidence as the basis for evidence-based decision making: is strength of evidence absolute?

Background One of the most challenging aspects of evidence-based decision making is the appraisal of the available evidence in order to separate the wheat from the chaff. It has been stated that “not all evidence is created equal” (Cochrane Consumer Network Levels of evidence), and it has been recommended to grade the available literature by the strength of evidence as determined by the methods used to minimize bias within a study design (Cochrane Consumer Network Levels of evidence; Bossuyt & Leeflang 2008; Gutmann 2009; Mileman & van den Hout 2009; Reitsma et al. 2009; Rosenberg & Donald 1995; Suebnukarn et al. 2010; Sutherland & Matthews 2004; Zwahlen et al. 2008). A cornerstone of this appraisal process is the use of hierarchical systems for classifying the evidence. This hierarchy is known as the Levels of evidence (Burns et al. 2011).