Statistics are statistics, but different disciplines use different terminology to refer to the same types of analysis. A 2013 Institute of Medicine Report on cancer care lists a number of different study types (see below). Whereas economists would refer to a study that examines groups with and without an event or outcome as a “difference-in-difference” study, in the medical literature these are cohort studies. Fortunately, there is some overlap. Both economists and medical researchers have the same definition for a cross sectional study.
Experimental study: A study in which the investigators actively intervene to test a hypothesis.
- Controlled trials are experimental studies in which a group receives the intervention of interest while one or more comparison groups receive an active comparator, a placebo, no intervention, or the standard of care, and the outcomes are compared. In head-to-head trials, two active treatments are compared.
- In a randomized controlled trial (RCT), participants are randomly allocated to the experimental group or the comparison group. Cluster randomized trials are RCTs in which participants are randomly assigned to the intervention or comparison in groups (clusters) defined by a common feature, such as the same physician or health plan.
Observational study: A study in which investigators simply observe the course of events.
- In prospective observational studies, the exposure of interest is studied using data stored in registries, which can require years to accumulate the needed numbers of patients and outcomes.
- In cohort studies, groups with certain characteristics or receiving certain interventions (e.g., premenopausal woman receiving chemotherapy for breast cancer) are monitored over time to observe an outcome of interest (e.g., loss of fertility).
- In case-control studies, groups with and without an event or outcome are examined to see whether a past exposure or characteristic is more prevalent in one group than in the other.
- In cross-sectional studies, the prevalence of an exposure of interest is associated with a condition (e.g., prevalence of hysterectomy in African American versus white women) and is measured at a specific time or time period.
Systematic review (SR): A scientific investigation that focuses on a specific question and that uses explicit, planned scientific methods to identify, select, assess, and summarize the findings of similar but separate studies. It may or may not include a quantitative synthesis (meta-analysis) of the results from separate studies.
- A meta-analysis is an SR that uses statistical methods to combine quantitatively the results of similar studies in an attempt to allow inferences to be made from the sample of studies and applied to the population of interest.
- Institute of Medicine. Delivering High-Quality Cancer Care: Charting a New Course for a System in Crisis. Released. September 10, 2013. Consensus Report