Study Design Classification

Randomized controlled trials 
Study subjects (individuals or communities or groups) are randomly assigned to 2 different groups (usually intervention and control, but may be 2 or more interventions) and outcomes are compared. Solomon Four-Group Design and Post-Test only Group design with randomization, and experimental studies are variations of RCTs. Clinical trials and community trials can be RCT’s. 

Controlled trials that were not randomized 
A common example would be a trial within a medical practice in which the practitioner gave the intervention to all families seen in April and May and used all families seen in June and July as the controls. Or a surgical trial in which patients given the new treatment are compared with a previous series of cases given the old treatment. Or surgeon A used the new technique and compared results with surgeon B, who used the old method. Although the assignment to one trial arm or another is not random, the assignment is controlled by the investigator(s). The intervention is applied to people one-at-a-time, and so the unit of the analysis is the individual. Some quasi-experimental designs would be included in this category. Clinical trials and community trials can be in this category. 

Cohort study 
In a cohort study, members of a defined population receive different exposures and their outcomes are compared. Assignment of exposure is not in the hands of the investigator. It may be decided by the individual (to smoke or not to smoke) or by other circumstances. In a cohort study, we have at least some information about people at the level of the individual. These are a type of observational study; also known as longitudinal study design. Cohort studies can be prospective or retrospective. 

Case-control study 
Cases are persons with the outcome of interest, and controls are drawn from the same population that produced the cases. The exposure of both groups is then compared. This is also a type of observational study. Case-control studies can be prospective or retrospective. 

Ecologic study 
The intervention of interest is applied across an entire population or group. The investigator has no control over the intervention. Outcomes are then measured in the group(s) which received the intervention and the group(s) which did not receive the intervention. The unit of analysis is the group. These studies generally fall into 2 categories, which can be combined in a single study: 

A. Ecological time study 
The first category involves one group and the analysis measures outcomes before and after the intervention is applied: one could call this, simply, a before-and-after study or a time-trend study. 

B.  Ecological group study 
The second category involves comparing two or more groups concurrently. 

C.  Ecological mixed study 
Both within-group changes are examined and between-group changes are examined. For example, we might compare traffic-crash mortality before and after passage of a seat-belt law in State A, and also compare this data to mortality in State B, which has no such law during the entire period being examined. Ecological studies may be called many different things by different authors and perspectives: time series studies, before and after studies, Pretest-Posttest studies, and others.

Case series 
Only cases are described. There is no control group. For example, the authors might report that 80% of all persons who died while riding a snow-mobile were intoxicated with alcohol. These studies are not included in our evaluations. 

Laboratory study 
This is a study which performed some type of test in something other than a human population. For example, the ability of various life-vests to keep a dummy afloat might be tested. These studies can be used in background section of the review.

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