Observational study

In fields such as epidemiology, social sciences, psychology and statistics, an observational study draws inferences from a sample to a population where the independent variable is not under the control of the researcher because of ethical concerns or logistical constraints.

The independent variable may be beyond the control of the investigator for a variety of reasons: "Although observational studies cannot be used to make definitive statements of fact about the "safety, efficacy, or effectiveness" of a practice, they can:[4] In all of those cases, if a randomized experiment cannot be carried out, the alternative line of investigation suffers from the problem that the decision of which subjects receive the treatment is not entirely random and thus is a potential source of bias.

One common approach is to use propensity score matching in order to reduce confounding,[5] although this has recently come under criticism for exacerbating the very problems it seeks to solve.

[9] A 2014 (updated in 2024) Cochrane review concluded that observational studies produce results similar to those conducted as randomized controlled trials.

[10] The review reported little evidence for significant effect differences between observational studies and randomized controlled trials, regardless of design.