Types of research articles and the hierarchy of scientific evidence

Not all evidence is created equal. At least that is the premise of the hierarchy of scientific evidence: a simplified tool used to understand how different types of articles and reports compare in terms of the strength of evidence provided.

It is a tool created for the medical field, and, while many variations exist, they boil down to the same idea: some pieces of evidence, articles, and methodological approaches are of better quality (particularly for clinical application) than others.

In medicine, the top of the hierarchy is systematic reviews and meta-analyses. In other words, studies quantifying or collating the compound outcomes of a bunch of smaller studies. But not just any smaller studies! Ideally, these will be of randomised controlled trials, which are highly controlled experiments where research outcomes are often trusted, but, due to practicality, sample sizes are often small and individual study outcomes may be unclear.

Right at the bottom of the hierarchy are case reports of individuals and expert opinions. That is not to say they are useless!

Case reports are detailed descriptions of patient symptoms and outcomes and are essential in the context of incredibly rare diseases or situations. Publishing on these situations means that others in medicine can find and compare with their own unusual cases. This creates the opportunity to better understand, diagnose, or treat something that is exceptionally hard to research using other methods.

Similarly, expert opinions or reviews are highly valuable at unpacking a knowledgeable perspective of a field at a given time. They often include calls for specific avenues of research or theoretical suggestions. However, they do not present systematic and traceable "evidence" in the same way as many of the other articles do. In this way, it is lower quality than other types of research in terms of having a "final say".

The Hierarchy of Evidence

As much as the pyramid is designed for medical research, other fields are starting to adapt it to their own types of research.

If you are a student or early career researcher, it is worthwhile being aware of this hierarchy to help you compare the quality of claims you may see in the literature. This is especially the case if you see ambiguous outcomes across studies. A good comparison helps you write more critically.

The pyramid does have its fair share of cynics, though (obviously... this is research). And, personally, I have seen some rubbish published as systematic reviews and some insightful qualitative research. Execution and rigour will always outweigh the type of article alone and where it fits on the pyramid.

However, it is a useful tool to begin your comparisons. The "types of research articles" you encounter provide a useful first step if you do not know where else to start.

Side note: Not everything even belongs on the pyramid. Some kinds of research, especially if unpublished or without peer review, may be useful and of relatively good quality, up-to-date, and worth including in your own literature review. This is known as "grey literature".

For more reading:

• Al Noman, A., Sarkar, O., Mita, T.M., Siddika, K. and Afrose, F., 2024. Simplifying the concept of level of evidence in lay language for all aspects of learners: In brief review. Intelligent Pharmacy, 2(2), pp.270-273. https://doi.org/10.1016/j.ipha.2023.11.002

• Evans, D., 2003. Hierarchy of evidence: a framework for ranking evidence evaluating healthcare interventions. Journal of Clinical Nursing, 12(1), pp.77-84. https://doi.org/10.1046/j.1365-2702.2003.00662.