Classification of Research ~Levels of Evidence

The classification of research and the terms ‘Levels of Evidence’ along with more recent ‘Evidence Based Guidelines’ were discussed as far back as the mid-1960’s and continued into the 1980’s. The conversation was an attempt to standardize and formalize the classification of research information used across disciplines. The process changed decisions about robust, reliable and valid research in the health sciences from the more common clinical judgment to a systemic process where all research is ranked according to the degree of the quality of the research using very specific guidelines.

There are many components used to ‘rank’ different types of research and it can be rather confusing without a comprehensive understanding of the types of research and the details of different methodological approaches to conducting research. As the SNJ Associates blog grows, we will cover each type of research method and study design in detail; however, for now, it is important to know that:

Not all evidence is the same!

Classification of Research

The table below is found widely across many sources with some slight variation to the detail included. For now, understanding that there are 5 main levels and within each level are particular types of studies is a good first step. Use the glossary provided at the Centre for Evidence-Based Medicine (CEBM) for specific definitions for each type of study mentioned in order to become familiar with the terminology. Also, a more detailed list from the CEBM is also provided below. The CEBM is located at the University of Oxford and the website is an excellent resource for evidence-based health research.

What you want to focus on is the ability to classify any study you are reviewing into it’s ‘type’ and then locate that type in the correct level. Once you can classify what you are reading, you can then determine what level of evidence is being presented. Any article written well will provide you all these details in the abstract or summary section.  

It is important to understand based on how the research information is derived, what possible bias may exist, what outcomes were studied and how the information is analyzed as each are key components that indicate the quality of research. Therefore, a systematic review or randomized control trial is considered better quality research when compared to a case-control study or an expert opinion. This does not mean that the research classified near the bottom is not good research rather it speaks to how strong the evidence is about the findings of the study. Whenever you are looking at research findings, always try and find the highest level of evidence available.


Table 1:1 Simplified Table of the Levels of Evidence

Level 1

  1. Systematic Review of RCTs with Meta-analysis
  2. Randomized Controlled Trial
Level 2

  1. Systematic Review of Cohort Studies
  2. Cohort Studies
  3. Before-After Studies
  4. Outcome Studies
Level 3

  1. Systematic Review of Case-Control Studies
  2. Case-Control Study
Level 4

  1. Cross-section studies
  2. Case-series
Level 5

Expert Opinion


A little more detail for each level:


The Centre for Evidence-Based Medicine, Oxford in 2009 Published the Follow Criteria:


  • 1a: Systematic reviews (with homogeneity) of randomized controlled trials
  • 1b: Individual randomized controlled trials (with narrow confidence interval)
  • 1c: All or none randomized controlled trials
  • 2a: Systematic reviews (with homogeneity) of cohort studies
  • 2b: Individual cohort study or low quality randomized controlled trials (e.g. <80% follow-up)
  • 2c: “Outcomes” Research; ecological studies
  • 3a: Systematic review (with homogeneity) of case-control studies
  • 3b: Individual case-control study
  • 4: Case-series (and poor quality cohort and case-control studies)
  • 5: Expert opinion without explicit critical appraisal, or based on physiology, bench research or “first principles


The whole classification of the levels of evidence in research has come under criticism. For example, one must be careful when comparing terms that are not universally applied or defined across research studies. However, the guiding principle of ranking the level of the evidence presented does provide great value and it is very important to have an overall understanding of the classification system when using research information.

At SNJ Associates, we believe having a basic understanding of the classification of evidence among research information in the health sciences is particularly important given the amount of data we are bombarded with on the Internet about our health and wellness. There are a plethora of ‘expert’ opinions online and regardless of how well written, presented and from whom they are posted by, the simple fact remains that an expert opinion is the lowest level of evidence.

Whenever you are presented with an expert opinion check their references, understand where their claims come from. Knowing the difference between an expert giving an opinion and summarizing reliable quality research is an excellent skill to master.  As an informed critical consumer of research information, you must distinguish between quality research information and clever marketing

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