Definitions of Study Types
Randomized Controlled Trial: A trial, in which participants are randomly assigned to two or more groups: at least one (the experimental group) receiving an intervention that is being tested and another (the comparison or control group) receiving an alternative treatment or placebo. This design allows assessment of the relative effects of interventions.
Meta-analysis: A statistical technique that summarizes the results of several studies in a single weighted estimate, in which more weight is given to results of studies with more events and sometimes to studies of higher quality.
Systematic Review: A review in which specified and appropriate methods have been used to identity, appraise, and summarize studies addressing a defined question. It can, but need not, involve meta-analysis. In clinical evidence, the term systematic review refers to a systematic review of RCTs unless otherwise specified.
Randomized Controlled Trial: A trial in which participants are randomly assigned to two or more groups; at least one (the experimental group) receiving an intervention that is being tested and another (the comparison or control group) receiving an alternative treatment or placebo. This design allows assessment of the relative effects of interventions.
Controlled Clinical Trial: A trial in which participants are assigned to two or more different treatment groups. In Clinical Evidence, we use the term to refer to controlled trials in which treatments are assigned by a method other than random allocation. When the method allocation is by random selection, the study is referred to as a randomized controlled trial (RCT). Non-randomized controlled trials are more likely to suffer from bias than RCTs.
Cohort Study: A non-experimental study design that follows a group of people (a cohort), and then looks at how events differ among people within the group. A study that examines a cohort, which differs in respect to exposure to some suspected risk factor (e.g. smoking), is useful for trying to ascertain whether exposure is likely to cause specified events (e.g. lung cancer). Prospective cohort studies (which track participants forward in time) are more reliable than retrospective cohort studies.
Case control study: A study design that examines a group of people who have experienced an event (usually an adverse event) and a group of people who have not experienced the same event, and looks at how exposure to suspect (usually Noxious) agents differ between the two groups. This type of study design is most useful for trying to ascertain the cause of rare events, such as rare cancers.
Definitions from the British Medical Journal's Clinical Evidence Glossary