A clinical study that is used to gather data but does not involve the testing of an intervention (e.g. new drug).

Observational studies are used to collect varying kinds of information, such as the prevalence of an infection, cause of an infection, use of particular drugs in clinical settings, the characteristics of affected patients, the care procedures used, and clinical outcomes for normal standard of care. The information gathered is typically used to identify patient populations and suitable sites for trials, and to guide the design of interventional studies, which assess the impact of interventions such as new drugs.

Observational studies can be used to generate evidence of the efficacy of treatments, for example through case–control designs (where outcomes in cases are compared with those in matched individuals). Evidence from observational studies is generally not seen as reliable as that from randomized controlled trials.