Pyramids are guides not rules: the evolution of the evidence pyramid

Soon after the inception of the evidence-based medicine (EBM) movement in the 1990s, the evidence pyramid rose from the sand.1 As more evidence resources and resources for finding evidence were developed it became necessary to provide guidance on their use. The original ‘4S’ system (systems (eg, computerised decision support), synopses, syntheses, studies) soon evolved into the ‘5S’ system by adding summaries (‘evidence-based textbooks’), which incorporated information from the lower levels of the pyramid into a more comprehensive evidence base for managing a given condition.2 The latest evolution of this model by Haynes and colleagues, the ‘6S’ model, recognised that not all synopses (succinct, structured summaries of clinically important, methodologically sound studies published in evidence-based journal like this one) are equal, and synopses were further divided into two levels (synopses of studies and synopses of syntheses).3 A further complexity of these pyramids is a hierarchy or pyramid of individual studies, which are located at the base of the ‘4S’ through ‘6S’ pyramids. This pyramid within a pyramid begins with in vitro studies at its base followed by ideas and opinions, case reports and case series, case–control studies, cohort studies, randomised controlled trials (RCT), and culminating with systematic reviews/meta-analysis at its apex.4 Inherent in all of these pyramids is the concept (while not always true) of a hierarchy; that less valid (higher risk of bias) evidence is at the bottom of the pyramid and more valid at the top. Also …