An effectiveness study is designed to study whether a therapy or intervention produces the expected therapeutic effect in real world clinical practice or under circumstances that more closely approach real-world practice. Many phase 4 clinical trials are effectiveness studies.
Elements of an effectiveness study that differ from an efficacy study (phase 1, phase 2, and phase 3 clinical trials) include less restrictive or no eligibility criteria (outside the approved label indications), less-standardized treatment protocols (allowing a doctor to adjust dose or combine with other approved therapies that are not contraindicated on the approved label), and delivery in a participant’s normal place of clinical care (for example, in a doctor’s office, clinic, outpatient service, or hospital depending on how the therapy is delivered). In effectiveness study, the doctors (or appropriate medical or allied health professional) and staff giving the therapy are not necessarily experts in the specific disease or therapy. Effectiveness studies usually also track adverse events and serious adverse events.
Criteria for Distinguishing Effectiveness From Efficacy Trials in Systematic Reviews [Technical Reviews, No 12. Agency for Healthcare Research and Quality (US), 2006 Apr]
A Primer on Effectiveness and Efficacy Trials [2014 published article]