Choice as an alternative to control in observational studies

In a randomized experiment, the investigator creates a clear and relatively unambiguous comparison of treatment groups by exerting tight control over the assignment of treatments to experimental subjects, ensuring that compar able subjects receive alternative treatments. In an observational study, th e investigator lacks control of treatment assignments and must seek a clear comparison in other ways. Care in the choice of circumstances in which the study is conducted can greatly influence the quality of the evidence about treatment effects. This is illustrated in detail using three observational studies that use choice effectively, one each from economics, clinical psy chology and epidemiology. Other studies are discussed more briefly to illus trate specific points. The design choices include (i) the choice of researc h hypothesis, (ii) the choice of treated and control groups, (iii) the expl icit use of competing theories, rather than merely null and alternative hyp otheses, (iv) the use of internal replication in the form of multiple manip ulations of a single dose of treatment, (v) the use of undelivered doses in control groups, (vi) design choices to minimize the need for stability ana lyses, (vii) the duration of treatment and (viii) the use of natural blocks .