Chatu Yapa, Masters of Applied Epidemiology
You may have heard of the Food Pyramid, the Population Pyramid and maybe even the Great Pyramids of Egypt. But have you heard about the Evidence-Based Medicine Pyramid (Figure 1)?
Evidence based medicine has been described as ‘the conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients.’ The Evidence-Based Medicine Pyramid describes the different levels of evidence that can be used to make health-related decisions. It helps us put the results of each study design into perspective and indicates that relative weight can be attributed to a particular study design.
Generally the higher up a methodology is ranked, the more robust it is assumed to be. In other words, as we ascend through these different study designs, we become more confident that their results are accurate, have less chance of statistical error, and minimize bias from confounding variables that could have influenced the results.
At the base of the pyramid is anecdotal evidence based on expert opinion and/or beliefs. Although this information is valuable and does have a role in decision-making, it lacks scientific validity and vigour. Case-series, case-control and cohort studies are observational studies and represent the first stage of testing an observation. Cohort studies normally look at the effect of suspected risk factors that cannot be controlled experimentally – for example, the effect of smoking on lung cancer. They usually follow a large group of people over an extended period of time. Results from these studies are usually more reliable and generalizable than case-control studies which look retrospectively at a group with illness/intervention and compares them to similar group who did not have the illness/intervention.
The true experimental design is the Randomized Control Trial (RCT), where individuals are randomly allocated into two or more groups. One group receives the intervention under investigation and the other(s) receives no treatment, a placebo, or a routine intervention.
Moving up the pyramid, we come to critically appraised topics. These are not study designs, but are short summaries of the best available evidence. At the pinnacle of the Evidence-Based Medicine Pyramid are systematic reviews and meta-analyses. Systematic reviews aim to assess and compare multiple studies to draw conclusions while meta-analyses review multiple studies and make a statistical summary of the effects of interventions. Systematic reviews and meta-analyses are generally considered to provide the highest quality of evidence.