Evidence is evidence, right?
There is a hierarchy of evidence, from weakest to strongest, that help folks make decisions about science and medicine.
Hierarchy of Evidence
For any study, you have to review and judge the quality of the evidence it provides.
Is it a case report (a glorified anecdote), case series, or animal study (lowest quality evidence)?
Or a systemic review or meta-analyses (highest quality evidence)?
“The first and earliest principle of evidence-based medicine indicated that a hierarchy of evidence exists. Not all evidence is the same. This principle became well known in the early 1990s as practising physicians learnt basic clinical epidemiology skills and started to appraise and apply evidence to their practice. Since evidence was described as a hierarchy, a compelling rationale for a pyramid was made.”
Murad et al. on the New Evidence Pyramid
What about case control studies, cohort studies, and randomized controlled trials?
They lie somewhere in between on the hierarchy of evidence scale or pyramid.
And there are other factors to consider when judging the reliability of a study.
“Ultimately, the interpretation of the medical literature requires not only the understanding of the strengths and limitations of different study designs but also an appreciation for the circumstances in which the traditional hierarchy does not apply and integration of complementary information derived from various study designs is needed.”
Ho et al. on Evaluating the Evidence
For example, you might also have to take into account the sample size of the study.
A study can be underpowered if it doesn’t have enough subjects. Unfortunately, even an underpowered study will give you results. They likely won’t be statistically significant results, but folks don’t always realize that.
Even a meta-analysis, usually considered to be at the top of the hierarchy of evidence pyramid, can have problems that make their results less useful, such as not using appropriate inclusion criteria when selecting studies and leaving out important studies.
All in all, there are many factors to look at when reading a medical paper and considering if the results are valid and should influence what you do and how you think. This is especially true when looking at low quality vaccine papers, many of which the anti-vaccine movement uses to scare people, even though they are often poorly designed, and several of which have been retracted.
What to Know About the Hierarchy of Evidence
Learning about the hierarchy of evidence can help you better evaluate medical studies and vaccine papers and understand that there is more to doing your research about vaccines than searching PubMed and reading abstracts.
More on the Hierarchy of Evidence
- The hierarchy of evidence: Is the study’s design robust?
- The hierarchy of evidence: Levels and grades of recommendation
- Understanding Research Study Designs
- New evidence pyramid
- Evaluating the Evidence
- How to Read a Clinical Trial Paper
- How to Evaluate the Literature
- Critical Appraisal of Scientific Articles
- A Simple Method for Evaluating the Clinical Literature
- Cochrane Handbook for Systematic Reviews of Interventions
- Not all evidence is created equally
- Hierarchy of scientific evidence – keys to scientific skepticism
- Evidence-Based Practice
- Oxford Centre for Evidence-based Medicine – Levels of Evidence
- GRADE working group
- GRADE: an emerging consensus on rating quality of evidence and strength of recommendations
- Study – Hierarchy of evidence: a framework for ranking evidence evaluating healthcare interventions.
- Study – When should we change our clinical practice based on the results of a clinical study? The hierarchy of evidence.