The different levels of scientific evidence

In recent centuries, science has advanced at a forced pace. Various studies on different subjects are conducted at the same time and in different parts of the world and every so often articles and results of experiments come to light, sometimes contradictory between Yeah. But not everything that comes to see the light has the same level of evidence.

Some of the results and experiments come from single case investigations, others though they have done research exhaustive only assess their own results, others start from mere observation... That is why we must take into account the existence of different levels of scientific evidence. These are precisely the topic that we are going to talk about in this article.

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

It is understood as level of scientific evidence the degree of scientific rigor that a certain study has or possesses, its results being more or less reliable and contrasted and its implications having been analyzed more or less systematically. Among the elements that make it possible to determine it, we can find how the data was obtained, if there is a risk of subjectivity or misinterpretation, if they have been reviewed systematically the data through statistical measures of reliability, validity or effect size or if different studies have been contrasted to reach the conclusions finals.

It is something that allows us to act based on the evidence available to date, something 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 can be contradictory. Not choosing correctly could not produce the improvement that is sought or not exploit the possibilities of improvement to the maximum and in the most efficient way, or even cause harm. In this way, knowing the level of evidence can serve to establish degrees of recommendation of 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 as a cure) for many diseases) is good for treating the black plague, when in reality it would be causing the defenses of the body to decrease. patient. But if it is contrasted with the use of antibiotics, the professional decides more for this last option as it is more effective.

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Two concepts to keep in mind

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

Systematic review

A systematic review is the collection and joint analysis of different investigations related to the same topic to be studied. A systematic analysis of the primary trials obtained is carried out and the data obtained are evaluated and contrasted. It is transparent and performs a thorough review of the material provided, but nevertheless a statistical analysis of these is not performed.

meta-analysis

A meta-analysis is understood to be that document in which a review of the research is carried out. carried out on a specific subject, checking and contrasting the data reflected by the different rehearsals and performing the statistical analysis of the effect size. It could be understood as a systematic review carried out with quantitative procedures, in such a way that so 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 a higher level of scientific evidence, if it is well done.

Different classifications of levels of scientific evidence

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

NICE and SIGN classification

Next we expose one of the most well-known and used scales to assess the levels of scientific evidence, that of the National Institute for Health and Clinical Evidence or NICE. Regarding the study related to the effectiveness of a therapy, the 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. Are meta-analysis of a higher quality, systematic reviews of randomized controlled trials or conducted studies and randomized controlled trials. With very low risk of bias.

1+

This level gathers the meta-analyses, systematic reviews or clinical trials that have characteristics similar to the previous one but being the control carried out less systematized and there is a little more risk of error.

1

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

2++

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

2+

Systematic reviews and well-conducted cohort or case-control studies with low risk of bias and with moderate probability of establishing causal relationships. At least one clinical trial or prospective controlled study without randomization exists.

2

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

3

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

4

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

OCEBM: Oxford Classification of Scientific Evidence Levels

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

1a

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

1 B

Controlled cohort studies with a high level of follow-up, which validate the quality with reference standards in aspects such as diagnosis.

1 C

These are those studies that reflect the efficiency and effectiveness based on clinical practice, taking into account different variables and possessing high specificity. However, it has not been verified through cohort studies.

2a

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

2b

The studies included at this level are usually cohort, with incomplete follow-up and no quality controlled trials. Also retrospective studies and studies that are limited to reviewing the available evidence.

2 C

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

3rd

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

3b

This level groups individual case and control studies, in which an objective analysis based on a reference standard is carried out, but which not performed on all study subjects. Those carried out without said standard are also included.

4

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

5

The lowest level of scientific evidence is based solely on expert opinion without there being an evaluation or a specific job, being rather grounded in theory.