Invited Commentary: Assessing treatment effects by using observational analyses--opportunities and limitations.

Making decisions about medical treatments based upon valid evidence is critical to improve health-care quality, outcomes, and value. Although such research commonly connotes the use of randomized controlled trials, experimental methods are not always feasible, and research using observational, quasi-experimental, and other nonexperimental methods may also be important. At the same time, nonexperimental methods are inherently susceptible to various types of bias and thus present special challenges in the search for valid and generalizable evidence. The study by Gardarsdottir et al. (Am J Epidemiol. 2009;170(3):280-285), on which this commentary is based, addresses a key potential source of bias -- mismeasurement of patients' duration of treatment -- in previous research on pharmacotherapy for depression. However, the authors' study is unlikely to address other potential sources of bias, which may make interpretation of their findings more difficult.