An observational study is an empiric investigation of the effects of a treatment, policy, intervention or exposure which was not randomly assigned to subjects, as it would be in a randomized experiment. Observational studies are common in most fields that study people, because harmful or unwanted treatments cannot be imposed on human subjects for experimental purposes. The central difficulty in an observational study is that, because treatments were not randomly assigned, the subjects receiving different treatments may not be comparable, so differing outcomes after treatment may not be effects caused by the treatment. If the treatment groups differ before treatment in ways that have been measured, there is an overt bias that often can be removed by adjustments, such as matching. Often there is concern that treatment groups differed before treatment in ways that were not measured, that is, concern about hidden biases. Hidden biases cannot generally be removed by adjustments and must be addressed by other means. Statistical methodology for observational studies concerns adjustments for overt biases and methods of design and analysis to address hidden biases. The current project will develop and extend statistical methods for overt and hidden biases.
In many fields, including health services research, epidemiology, public program evaluation, and economics, most studies of treatment effects are observational studies. Observational studies are also common in several fields that perform experiments, including clinical medicine and psychology. Statistical methods for observational studies are useful and are used in these fields.