No le creas a todos los expertos: Jerarquía de evidencia científica

Don't believe all the experts: Hierarchy of scientific evidence

by Baia Food on Jan 17, 2019

Science is the tool that has allowed human beings to understand the world around them and be able to apply everything learned to benefit.

Advances in science are fundamental as they influence all aspects of life, from finding cures for diseases to discovering solutions to environmental challenges and being able to live sustainably.

Science is neither bad nor good in itself; this will depend on its interpretation and the purposes for which it is used. On the other hand, it will ALWAYS be necessary to approach science as a society to have the necessary knowledge and choose between all the options we have, whether personal, political or professional.

At Baïa we want to invite you to come closer to the fascinating world of research.

With so much information available today it is dangerous to rely on erroneous opinions and data and that is why we want to teach you how to look for the best evidence available.


Scientific evidence is classified depending on the design of the studies from which it comes.

This facilitates the search for the best available evidence since it gives it a hierarchical value that works as a guide to decide. Based on this evidence, a recommendation should be made based on the strength of the evidence that supports it.

In 1979, the Canadian Task Force on the Period Health Examination created the first hierarchy of evidence for the evaluation of preventive measures.

Since then there has been a great increase in proposals and classifications to prioritize the evidence and its degrees of recommendation. There are 11 proposals that are the most used today (CTFPHC, Sackett, USPSTF, CEBM, GRADE, SIGN, NICE, NHMRC, PCCRP, ADA and ACCF/ AHA) but in order not to confuse you, we will summarize Sackett's proposal.


This classification was proposed by epidemiologist David L. Sackett and is the one generally used. Rank the evidence from 1 to 5, with 1 being “the best” evidence and 5 being “worse or less good.”

Most experts agree that the higher up the pyramid, the less likely the methodology and results are to be biased.

Meta-analyses and systematic reviews are at the top of the pyramid while expert opinions and anecdotal experience are at the bottom.

  1. Systematic reviews are characterized by using a scientific, transparent and detailed methodology in which the studies and evidence that is available on a specific question are collected, reviewed and evaluated and the results are summarized. A meta-analysis differs from a systematic review in that it uses statistical methods to combine the results of studies.
  2. Randomized controlled trials also known as randomized clinical trials. Participants are randomly assigned into separate groups to compare treatments or interventions, one being the control which can be a placebo, no intervention or the standard intervention.
  3. Cohort trials are a type of observational research with the purpose of determining the association between exposure factors and development of the event of interest both prospectively and retrospectively (natural history, survival, factors and prognoses).
  4. Case-control studies compare two groups of people: one with the disease or condition and another similar group who do not have the disease or condition and are compared. It is different from a cohort study since in this case the individuals are chosen according to the disease and the past is observed and in a cohort study they are selected according to the exposure and we wait to see if it develops or not. the disease or condition.
  5. Cross-sectional studies are observational studies where variables collected at a specific time on a specific population are analyzed.
  6. Editorials and expert opinions: in medical-scientists, the personal opinion of professionals should be of little importance since trusting personal opinions is dangerous, especially if they claim to be "experts" without scientific studies to support it.


You would be lying to yourself if you thought that for every question you have you were going to go find scientific evidence and classify it according to Sackett (although let me tell you, this would be an incredible idea).

What we want you to take away is the importance of basing ourselves on data and scientific studies with maximum evidence. Nowadays there are "experts" on every corner and many times we listen to them and take their information as law when they truly occupy the lowest place within Evidence-Based Medicine.

As long as we continue to base our knowledge on opinions and consensus and ignore clinical trials and meta-analyses, we will be acting in an outdated, uninformed and sometimes even dangerous way.

Don't stay with "I talked to a doctor about it and he says that this is dangerous for your health" or "I talked to an expert about it and he says that this is the absolute cure."

Before we had to rely on animal models, experiences, anecdotes and evolutionary logic but with all the evidence we have today and quality studies, we cannot settle for the above.

If you have any questions or want to contribute something, we'll wait for you in the comments!