An observational study is an empiric investigation of the effects caused by a treatment, policy, or intervention in which it is not possible to assign subjects at random to treatment or control, as would be done in a controlled experiment. Observational studies are common in most fields that study the effects of treatments on people.
The second edition of Observational Studies is about 50% longer than the first edition, with many new examples and methods. There are new chapters on nonadditiv ...