Aug 20 / 2020

The different levels of scientific evidence

A hierarchy of the different degrees of scientific support for research according to its type.

In recent centuries, science has been advancing at a rapid pace. Various studies on different subjects are carried out at the same time and in different parts of the world, and articles and results of experiments, sometimes contradictory to each other, come to light every few years. But not everything that comes to light has the same level of evidence.

Some of the results and experiments come from investigations of single cases, others, although they have carried out an exhaustive investigation, only evaluate their own results, others start from mere observation... That is why it is necessary to take into account the existence of different levels of scientific evidence. Precisely these are the subject of which we are going to speak in this article.

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

The level of scientific evidence is understood as the degree of scientific rigor that a given study has or possesses, its results being more or less reliableThe results are more or less reliable and contrasted and their implications have been more or less systematically analyzed. Among the elements that make it possible to determine this are how the data have been obtained, whether there is a risk of subjectivity or misinterpretation, whether the data have been systematically reviewed through statistical measures of reliability, validity or effect size, or whether different studies have been contrasted to reach the final conclusions.

This is something that allows us to act based on the evidence available so far, which is very relevant when making decisions. A typical example of this reason is that of medicine or psychology: when you have to decide which treatment is best for a patient with specific characteristics, choosing based on some studies or others is important, taking into account that the data may be contradictory. Not making the right choice may not produce the improvement you are looking for or may not exploit to the maximum and in the most efficient way the possibilities of improvement, or even generate harm. Thus, knowing the level of evidence can be used to establish degrees of recommendation for different therapies. establish levels of recommendation for different therapies and treatments..

For example, a study may say that bloodletting (understood as the extraction of Blood that was used in the Middle Ages as a cure for many diseases) is good for treating the Black Death, when in fact it would be causing the patient's defenses to be reduced. But if it is contrasted with the use of antibiotics, the professional may decide in favor of the latter option because it is more effective.

Two concepts to keep in mind

To correctly understand the relevance of each of the levels of scientific evidence, it is necessary to know beforehand some terms referring to the type of studies that are being ranked. Among them, the following two stand out:

Systematic review

A systematic review is the collection and joint analysis of different research studies related to the same topic to be studied.. A systematic analysis of the primary tests obtained is carried out and the data obtained are evaluated and contrasted. It is transparent and performs an in-depth review of the material provided, but no statistical analysis is performed.

Meta-analysis

Meta-analysis is understood as a document in which a review of the research carried out on a specific topic is performed, checking and contrasting the data reflected by the different trials and performing a statistical analysis of the effect size. statistical analysis of the effect size.. It could be understood as a systematic review carried out with quantitative procedures, in such a way that the data obtained from it are intended to be objective, systematized, precise and replicable. Technically, it is the type of document that usually has the highest level of scientific evidence, if it is well done.

Different classifications of levels of scientific evidence

Different studies and organizations (especially those linked to the world of health) have tried to create a hierarchy that organizes the different investigations based on the level of scientific evidence. In fact, there are a large number of different hierarchies of scientific evidence, there are a large number of different hierarchiesbut they are all broadly similar and refer to practically the same points.

NICE and SIGN classification

The following is a description of one of the best known and most widely used scales for assessing levels of scientific evidence, that of the National Institute for Health and Clinical Evidence (NICE).the National Institute for Health and Clinical Evidence or NICE. With regard to the study related to the effectiveness of a therapy, NICE uses the criteria and categories already proposed by the Scottish Intercollegiate Guidelines Network or SIGN. Specifically, the following levels of evidence are proposed

1++

These are studies with the highest level of scientific evidence. They are meta-analyses of superior qualitySystematic reviews of randomized controlled trials or studies conducted and randomized controlled trials. With very low risk of bias.

This level includes meta-analyses, systematic reviews or clinical trials that have similar characteristics to the previous one, but with less systematized control. less systematized control and with a somewhat higher risk of error. and with a somewhat higher risk of error.

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

2++

This level refers to systematic reviews of very high quality, with cohort and/or case-control studies, which have a very low risk of error.which have a very low risk of bias and a high probability of establishing causal relationships.

Systematic reviews and well-conducted cohort or case-control studies, with low risk of bias and a moderate likelihood of establishing causal relationships. moderate probability of establishing causal relationships. At least one clinical trial or non-randomized prospective controlled study is available.

In general, this level includes studies with a high risk of bias and with a high probability that the data and variables analyzed do not have a causal relationship.

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

These studies have not carried out an analysis per se, but rather limited themselves to collecting the opinion of experts in the field, without conducting without conducting experiments or collecting empirical data.

OCEBM: Oxford Classification of Levels of Scientific Evidence

In addition to the previous one, another of the most commonly used classifications is that made by Oxford, this being a modification based on another generated by Sackett. This classification is especially useful because it integrates levels of scientific evidence in different aspects, in treatment, diagnosis, prognosis, epidemiology and even economic studies. and even economic studies. The levels of evidence are, however, practically identical to the previous one.

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-upthat validate quality with reference standards in aspects such as diagnosis.

These are studies that reflect efficiency and effectiveness based on clinical practice, taking into account different variables and with high specificity. possessing high specificity. However, it has not been proven through 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 usually cohort studies, with incomplete follow-up and no quality-controlled without quality-controlled trials. Also retrospective studies and studies that are limited to reviewing the available evidence.

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

This level includes systematic reviews of cases and controls with homogeneity (i.e., the chosen literature maintains similar levels of effectiveness and there are no major discrepancies between the effects and characteristics of the studies used).

This level groups individual case-control studies, in which an objective analysis based on a reference standard is carried out, but which is not carried out in all studies. is not carried out on all study subjects.. It also includes those conducted without such a standard.

This level of evidence is one of the lowest given that a powerful analysis is not performed. These are generally case studies, cohort studies and low-quality case-control studies.

The lowest level of scientific evidence is based solely on the opinion of experts without any evaluation or concrete work, being rather based on thebeing rather based on theory.

(Updated at Apr 13 / 2024)