Unfortunately, a number of systems exist for categorizing types of studies. They often overlap to some degree, potentially causing the reader confusion. One such system is by time frame. In general, clinical studies are cross-sectional or longitudinal; although additional study types exist. Cross-sectional studies involve observations made at one point in time (a snapshot in time) and are often used for epidemiologic purposes. This study type is often used to establish a relationship or association between two or more variables. A disadvantage of this design is that cause and effect cannot be determined. Longitudinal studies involve observations over time. This may provide an opportunity for an intervention and a subsequent analysis of cause and effect.

In addition, studies may be classified as either retrospective or prospective. Retrospective studies collect data from written material such as medical records created before the study was designed or by subject recall. For this reason, verifying the existence of a risk factor or outcome condition to the same degree as with a prospective study design is difficult. As in cross-sectional studies, establishing a cause and effect relationship with this design is difficult. These studies are subject to recall bias as well as selection bias. The advantage of a retrospective design is that it is well suited to study rare diseases or conditions. Retrospective studies also serve to identify problems for subsequent prospective trials (hypothesis-generating). Prospective studies follow subjects forward in time and collect data as they are generated. Prospective studies may be interventional or observational.

Besides the element of time, studies may be classified by whether the investigator began with an outcome of interest, such as a disease, or an exposure of interest, such as a motor vehicle collision. Studies that begin with exposures usually follow a cohort of subjects with and without the exposure and are usually prospective. Studies that begin by identifying subjects with and without a particular disease (cases and controls) are often retrospective. However, this is an approximate, not an exact, rule.

A third type of study categorization is by degree of intervention or control over variables that can affect the outcome. Interventional design generally involves the evaluation of a specific therapy administered to each patient in a study group. Ideally, the intervention is withheld from another group (the control group), and direct comparisons of outcome are made. In some studies, withholding a specific intervention is not possible (eg, if it is standard of care), and these studies are termed uncontrolled. In the controlled trial, assignment of patients to either the interventional group or the control group should be random.

The prospective, randomized, controlled clinical trial is the criterion standard for making determinations of cause and effect or the value of a specific intervention. Although this is the strongest design, major limitations include expense; ethical problems inherent to testing new therapies or withholding them from control-group patients; and the time, money, and effort necessary to perform the study.

Observational design does not involve analysis of effects of an intervention but rather the effects of a specific characteristic shared by all members of the study group. A comparable control group of patients lacking the study characteristic permits a comparison of effect. Subjects are followed, without intervention, with respect to a particular outcome. Advantages of a prospective observational study include the ability to establish comparable subjects prior to beginning the study and the ability to follow them over time. This is important for such issues as risk factors associated with a particular disease or condition.