An observational study draws inferences 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.
A cross sectional study evaluates the risk factors and effect at a single point in time without any follow-up. It provides more evidence than case series and is located on the level of evidence (LOE) between the case series study and the longitudinal retrospective cohort study. A longitudinal study evaluates the patient at different times. The classification can be retrospective (case control study) and prospective (cohort study).
They are located on the level or quality of evidence between the cross-sectional studies and randomized clinical trial. In a retrospective cohort study, the outcome is measured at the end of the work, and the variables are verified in the database or patient records.
In a prospective cohort study, the outcome and the measures are defined at the beginning of treatment and the patients are followed to verify if the disease is positive or not.
see Case control study.