/Hierarchy of Evidence

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Not all evidence is created equal. Below is the basic pyramid of the hierarchy of evidence. At the top of the pyramid is Systematic reviews and meta-analyses of randomized controlled trials are at the top of the pyramid with expert opinion and editorials at the bottom.

Experimental Designs

Systematic Reviews

Systematic Review is a thorough and comprehensive search of the medical literature to answer a specific question. A good systematic review just like any other research should have a method of literature search and data abstraction set a priori with specific search strategy, process of selection, and inclusion/exclusion criteria for reviewed studies. Like all of evidence-based medicine, it should seek to answer a specific answerable question. A systematic review may contain a meta-analysis (see below) to help answer the question, however based on the available evidence a meta-analysis may not be able to completed.

In contradistinction to Systematic Review, non-systematic reviews or literature reviews are much less rigorous and not part of the pyramid of evidence. They are not necessarily based on a single question, but may be an overview or a series of questions on a topic. There is not necessarily a protocol or a specific objective and the results may not be summarized based on specific criteria or quality of evidence.

A Meta-Analysis is a specific type of systematic review in which a statistical tool is employed to pool available results from several similar studies into a single combined result. Thus, all meta-analyses are systematic reviews, but not all systematic reviews contain a meta-analysis

Randomized Controlled Trial

A randomized controlled trial is a trial that seeks again to answer a specific question by controlling as best as can be controlled for known variables and comparing results of an experimental group with a control group. Patients are enrolled in a randomized fashion to the experimental arm or the control arm. In general, the patients in the experimental arm receive the treatment being studied and standard care while the control arm receives placebo and standard care.


Observational Designs

Cohort Study

A Cohort Study evaluates the effect of a specific exposure (ie smoking) to find outcomes linked to that exposure. This design takes similar groups of patients (one exposed and one not exposed) and follows them forward in time to evaluate the effects of specific exposures on outcomes. The most famous cohort study is the Framinham Cohort study, which has elucidated much of what we know about cardiovascular risk over time and the classic cardiovascular risk factors.

While this may not be as powerful of a design as an RCT, an RCT cannot always be completed for every type of question. For example, we can’t randomize people to a smoking group and a non-smoking group. Cohort studies are relatively cheap compared to an RCT and thus can be carried out over many years. Of note, a cohort study can be done in a retrospective fashion accessing archived records, however the cohort study still starts with an exposure and follows the groups forward through time.

‘’As we develop large databases of clinical information in informatics, we may be able to leverage the retrospective cohort design to a considerable degree.’’

Case Control

A Case Control study, on the other hand, starts with a disease (i.e. cancer) and looks back in time for exposures that may have led to the disease or outcome. Its weakness is in the potential for recall bias in that we are asking patients to look back at all the many things they could have been exposed to. This design is particularly powerful though when an outcome takes a long time to develop or is very rare. There are some very powerful examples of well-done case control studies such as the identification of the link between smoking and lung cancer.

Case Series

A Case Series is a report of several observed cases or a collection of patients treated in a similar manner. There is no rigorous study design and no significant limitations on how this type of study is reported. Its goal is not necessarily to define a specific cause and effect relationship but to bring about awareness of possible associations. A case series may be written in order to highlight a possible link to spur further research on the topic. Other times a case series is published is because there is no other literature out there on a specific topic and this topic may be very rare. This is often the case for things like the effects of toxicological exposures or treatments for toxicologic exposures.

Case Report

A case report is a report of a single observation. This may be as simple as an interesting or notable case or it may be a finding where an association is suspected but there have been no other reports of this association in the literature.