The different levels of scientific evidence

Over the past few centuries, science has advanced at a rapid pace. Several studies are carried out on different subjects at the same time and in different parts of the world and from time to time articles and results of experiments, sometimes contradictory with each other, come to light. But not everything that comes to light has the same level of evidence.

Some of the results and experiences come from single case investigations, others although they have conducted a thorough investigation only value their own results, others are based on simple observation … That is why it is necessary to take into account the existence of different levels of scientific evidence. This is precisely the topic we will be talking about in this article.

    What is the level of scientific evidence and why is it so important?

    It is understood as a level of scientific proof the degree of scientific rigor that a particular study has or possesses, Being its more or less reliable and contrasting results and having analyzed more or less systematically its implications. Among the elements that allow us to determine how we obtained the data, whether there is a risk of subjectivity or misinterpretation, whether the data have been systematically reviewed through statistical measures of reliability, validity or size of the ‘effect or whether different studies were contrasted to reach the final conclusions.

    This is something that allows us to act on the basis of the evidence available so far, something very relevant when making decisions. A typical example of this reason is in medicine or psychology: when you need to decide which treatment is best for a patient with specific characteristics, choice based on one study or another is important, as the data may be contradictory. Failure to choose correctly may not lead to the improvement sought or exploit opportunities for improvement to the maximum and in the most effective manner, or even cause harm. In this way, knowing the level of proof can be used to establish levels of recommendation for different therapies and treatments.

    For example, a study may say that bleeding (understood as the extraction of blood that was used in the Middle Ages to cure many diseases) is good for treating the Black Death, when in fact it would cause a decrease in defenses. of the patient. But if she is opposed to the use of antibiotics, the same professional decides more so that the latter option is more effective.

      Two concepts to consider

      To fully understand the relevance of each level of scientific evidence, it is necessary to know in advance certain terms referring to the type of studies that are prioritized. These include the following two:

      Systematic review

      A systematic review is the collection and joint analysis of different research related to the same subject to be studied. A systematic analysis of the primary tests obtained is carried out and the data obtained is evaluated and compared. It is transparent and conducts a thorough review of the material provided, but nevertheless does not conduct a statistical analysis of it.


      Meta-analysis is understood to mean this document in which a review of the investigations carried out on a concrete subject is carried out, verifying and opposing the data reflected by the various tests perform a statistical analysis of the effect size. It could be understood as a systematic review carried out with quantitative procedures, so that the data obtained from it is supposed to be objective, systematized, exact and reproducible. Technically, this is the type of document that usually has the highest level of scientific evidence, if done right.

      Different classifications of levels of scientific evidence

      Different studies and organizations (particularly related to the world of health) have sought to create a hierarchy that organizes different research according to the level of scientific evidence. In reality, there are a large number of different hierarchiesBut basically they are all very similar and refer to practically the same points.

      NICE and SIGN classification

      Below we exhibit one of the best known and most used scales to assess levels of scientific evidence, That of the National Institute of Health and Clinical Evidence or NICE. Regarding the study on the effectiveness of a therapy, NICE uses the criteria and categories already proposed by the Scottish Intercollegiate Guidelines Network or SIGN. More specifically, the following levels of proof are proposed

      1 ++

      These are studies with the highest level of scientific evidence. they are high-quality meta-analysis, Systematic reviews of randomized controlled trials or studies and randomized controlled trials. With a very low risk of bias.


      This level groups together meta-analyzes, systematic reviews or clinical trials which have similar characteristics to the previous one but which are the control carried out less systematized and because there is a little more risk of error.


      We are talking about meta-analysis, systematic reviews or clinical trials with a high risk of bias.

      2 ++

      This level refers to very high quality systematic reviews, with cohort studies and / or cases and controls, Which present a very low risk of bias and a high probability of establishing cause and effect relationships.


      Systematic reviews and cohort studies or well-executed cases and controls, with low risk of bias and with moderate probability of being able to establish causal relationships. At least there is a non-randomized controlled trial or a prospective study.


      In general, this level includes studies at high risk of bias and with a high probability that the data and variables analyzed are not causally related.


      This level refers to studies that do not perform analysis. They are generally based on observation. Case reports would be a good example, as would correlational or case-control studies.


      These studies did not perform analysis per se, but rather they are limited to obtaining the opinion of experts in the field without performing experiments or collecting empirical data.

      OCEBM: Classification of Oxford Levels of Scientific Evidence

      In addition to the above, another of the most used classifications is that made by Oxford, this is a modification based on another generated by Sackett. This classification is particularly useful because it integrates levels of scientific evidence under different aspects, both in treatment and diagnosis, prognosis, epidemiology and even economic studies. The levels of proof, however, are virtually identical to those above.


      At this level of evidence, we find systematic reviews with homogeneity, with controlled and randomized studies, verifiable and contrastable in different populations.


      Controlled cohort studies with a high level of follow-up, Which validate the quality with benchmark standards in aspects such as diagnostics.

      1 C

      It is these studies that reflect the efficiency and effectiveness of clinical practice, taking into account different variables and possessing high specificity. However, it has not been verified by cohort studies.


      At this level, we mainly observe systematic reviews with homogeneity and generally including controlled or cohort trials.


      The studies included at this level are generally cohort, with incomplete follow-up and without controlled trials of quality. Also retrospective studies and studies limited to reviewing the available evidence.


      In general, this level refers to ecological studies and the search for health outcomes of different elements.


      this level includes systematic reviews of cases and controls with homogeneity (i.e. the literature selected maintains levels of efficacy and there is no major discrepancy between the effects and characteristics of the studies used ).


      This level groups together case studies and individual controls, in which an objective analysis based on a reference standard is carried out, but which it is not achieved in all subjects of the study. Also included are those made without this standard.


      This level of evidence is one of the lowest because no powerful analysis is done. These are typically case studies, cohort studies, and low quality case studies and controls.


      The lowest level of scientific evidence is based solely on expert opinion without any evaluation or concrete work, Be better anchored in theory.

      Bibliographical references:

      • Harbor, R. and Miller, J. (2001) A new system for classifying recommendations into evidence-based guidelines. BMJ 2001; 323: 334-6. Scottish Intercollegiate Guidelines Network Ranking Review Group.

      • Mella Sousa, M .; Zamora, P .; Mella Laborde, M .; Ballester, JJ and Uceda, P. (2012). Levels of clinical evidence and degrees of recommendation. Rev.S. and Traum. and Ort. 29 (½): 59-72.

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