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. One common observational study is about the possible effect of a treatment on subjects, where the assignment of subjects into a treated group versus a control group is outside the control of the investigator. This is in contrast with experiments, such as randomized controlled trials, where each subject is randomly assigned to a treated group or a control group.