Posted on April 29, 2014
You have seen the Pyramids of Egypt, you’ve heard of Maslow’s Pyramid of Needs and you’ve gotten more than your fill of the Food Pyramid.
Now, behold! The wonders of the Evidence-Based Medicine Pyramid!
The Evidence-Based Medicine Pyramid is simply a diagram that was created to help us understand how to weigh different levels of evidence in order to make health-related decisions. It helps us put the results of each study design into perspective, based on the relative strengths and weaknesses of each design.
Allow me to be your guide as we tour the Evidence-Based Pyramid.
As we approach, we can see the Evidence-Based Pyramid is divided into levels. Each ascending level represents a different type of study design and corresponds to increasing rigor, quality, and reliability of the evidence. 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.
The first level of the Evidence-Based Pyramid is the foundation. This background information is important and helpful. However, when viewed on its own, this type of evidence can be heavily influenced by beliefs, opinions, or even politics. This level might also include anecdotal evidence.
This level represents the first stage of testing an observation. Case Series reports usually include only a few participants who are given a similar intervention and follow-up. Case Control Studies are similar to case series, except it looks retrospectively at individuals and compares with a similar group who did not have the intervention. These studies are conducted in the early stages of research to help identify variables that might predict a condition. One of the weaknesses in these designs is that there are small numbers of participants and they are frequently not randomized or controlled for confounding variables.
Also called longitudinal or epidemiological studies, Cohort studies follow a large group of people over an extended period of time to see how their exposures affect their outcomes. This type of study is normally used to look at the effect of suspected risk factors that cannot be controlled experimentally – for example, the effect of smoking on lung cancer. These studies are frequently used to determine long term effects of a lifestyle, diet, or other interventions. Cohort studies may include a second group that did not engage in the same intervention as a control comparison. Although these studies are a step up in reliability and generalizability, they can be difficult to blind, can’t be controlled for outside variables, and are usually not randomized.
Now we have reached a major point in the Pyramid – the Randomized Control Trial, the true experimental design. In this study design, individuals are assigned by special randomization techniques into two or more groups, where one group receives the intervention under investigation and the other(s) receives no treatment, a placebo, or a standard intervention.
A large Double Blinded Randomized Control Trial is the most reliable “test” or study design and provides the strongest support of a cause and effect relationship. However, these studies are expensive and can be ethically problematic.
Critically appraised topics are not actually a study design. They are short summaries of the best available evidence. Basically, these are an abbreviated systematic review created to answer a specific question.
Now we have finally arrived at the pinnacle of the Evidence-Based Medicine Pyramid.
From this dizzying height, we take in a panoramic view of all of the evidence about an intervention. Systematic reviews take a bird’s eye view by comparing the results of studies side by side, typically on a forest plot. Systematic reviews are considered the strongest and highest quality of evidence.
We could also include meta-analysis here as well. This is where multiple studies are reviewed and a statistical summary is made that represents the effect of the intervention across multiple studies.
The Cochrane Collaboration takes systematic reviews to the next level. They are the experts of the systematic review and have an added a level of rigor as an independent voice, as well as developing special techniques to identify bias in studies.
Well, I hope you have enjoyed this tour of the Evidence-Based Pyramid. So next time you hear about a “study” in the media or anywhere else be sure to find out what kind of study design was and where it falls on the Evidence-Based Medicine Pyramid.
The pyramid was produced by HLWIKI Canada: http://hlwiki.slais.ubc.ca/index.php/File:EBMpyramid.gif
Some definitions were derived from Cochrane Summaries Lexicon page http://summaries.cochrane.org/lexicon/9#letter
Glover, J. (n.d.). Yale University Library Subject Guides. Evidence-Based Clinical Practice Resources. Pyramid. Retrieved April 10, 2014, from http://guides.library.yale.edu/content.php?pid=9786&sid=73113
Sedlacek, B. (n.d.). JML LibGuides. Evidence-Based Practice. Evidence Pyramid/Study Designs. Retrieved April 14, 2014, from http://libguides.methodistcollege.edu/content.php?pid=175181&sid=1474687