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Clinical Practice Guidelines: Recommendations for clinical care. Usually informed by a knowledge synthesis (systematic review) and clinical expertise.
Systematic Review: Synthesis of evidence (i.e. articles) on a defined topic/question. The group of researchers will evaluate and analyze the evidence to inform clinical practice.
Meta-Analysis: Usually follows a systematic review. Combines the data from multiple (similar) studies to address SR topic/question.
Randomized Controlled Trials: Assesses a specific intervention by comparing two similar groups, with one receiving the intervention and the other not (usually called a placebo). By doing this, researchers can assess the differences attributed to the intervention.
Cohort Studies: Observational studies regarding the association between a specific exposure or risk factor and the development of a clinical condition.
Case Control Studies: Observational studies regarding associations between a specific clinical condition and possible risk factors.
Cross-Sectional Studies: Observational studies that gather information at one point and time on the prevalence of health outcome(s), or exposure(s), or both. While absent from the CEBM Evidence Pyramid, they are usually considered to be at the same level as case control studies.
Case Reports: Report of a single patient, or treatment of several patients. There is no research design for case reports. They can be beneficial in identifying new health conditions.
Qualitative Research: Research that focuses on patient perspectives. Data can be in the form of art, recorded interviews, diaries, etc. Qualitative research compliments quantitative research by asking "why" or "how" questions and finding in-depth answers. While absent from the CEBM Evidence Pyramid, qualitative research is usually considered to be at the same level as case control studies.
Narrative/Literature Reviews: Overview of previous studies on a specific topic. Not as rigorous as a knowledge synthesis (i.e. systematic review) and there is no research design.
Editorials: Opinion piece in an academic journal that is not peer reviewed. No research design in an editorial.
| Level of Evidence | Strength | Grade |
|---|---|---|
| Level 1 Studies | Strong | A |
| Level 2, 3, or extrapolations from Level 1 | Moderate | B |
| Level 4 or extrapolations from Level 2, 3 | Limited/Weak | C |
| Level 5 or inconsistent, inconclusive studies from any level | Incomplete/Inconsistent | D |
(Table adapted from CEBM and Forrest, 2009)
There are many variations of the levels of evidence pyramid. You may find that qualitative studies are often excluded from these, but not because qualitative research is unimportant. The nature of clinical questions (therapy, diagnosis, etiology, and prognosis) is that they are best answered by quantitative evidence-- e.g. data that measures outcomes. Qualitative studies answer more in-depth questions relating to patient experiences, feelings, and perceptions.
While the CEBM Evidence Pyramid (pictured above) does not refer to qualitative studies, other groups place qualitative research below case reports but above case studies- nonexperimental, but still structured into an observational study design. Refer to this example from University of New Mexico. While it's not visually represented, qualitative research is typically regarded as a Level 3 on the CEBM Evidence Pyramid because it has design (Levels 4 and 5 do not).
It can also be helpful to think of levels of evidence specific to the question type. So, if the clinical question is about patient experience or perspective, there can be varying levels when it comes to qualitative research. The Joanna Briggs Institute ranks these similar to what we see in other pyramids, e.g. with systematic reviews of mixed-methods or qualitative studies at the top (see Levels of Evidence for Meaningfulness on p. 5).