Clinical researchers may use either interventional studies or observational studies. Clinicians may also conduct other types of clinical research including case reports, economic evaluations (as an independent research or an integrated part of observational studies or clinical trials), and systematic reviews and meta-analyses. Qualitative research is increasingly conducted within or alongside these clinical studies.
Each study design has advantages and disadvantages, but generally speaking some designs provide more reliable evidence than others. The following hierarchy is often used to indicate the strength of evidence provided by different clinical research designs.
The chart below provides guidance on thinking about the most appropriate research design.
The World Health Organization defines a clinical trial as any study that prospectively assigns human participants or groups of humans to one or more health-related interventions to evaluate the effects on health outcomes. Clinical trials can be non-randomized or randomized. In randomized trials, researchers randomly assign participants to different intervention groups and compare patient outcomes according to the intervention status. Go to the Clinical Trials section to learn more.
In observational studies, researchers do not assign any interventions, but observe what participants have received or were exposed to. Case-control studies and cohort studies are two commonly used observational study designs, but ecological studies and cross-sectional studies may also be used. Go to the Observational Studies section to learn more
There are several other types of clinical research designs that can be considered.
Case reports provide a detailed description of a limited number of patients, often without a comparison group. It is useful for hypothesis generation but not for hypothesis testing.
The study of the economic cost associated with a disease itself (e.g., direct medical cost of type 2 diabetes) and with interventions to treat the disease (e.g., cost-effectiveness of a new drug for managing type 2 diabetes). Go to the Economic Evaluations section to learn more.
The review systematically identifies relevant studies that answer a well-defined question and then synthesizes the evidence qualitatively or quantitatively. A meta-analysis will use statistical techniques to combine the synthesized data. Go to the Systematic Reviews and Meta-analyses section to learn more.
Qualitative research explores such things as ideas, opinions or problems in their natural setting to better understand them. It is increasingly used within or along with observational studies and clinical trials. Learn more about qualitative research.