The effectiveness of a therapy is not simply determined by its pure biological effect but is also influenced by the physician’s ability to administer, or the patient’s ability to adhere to, the intended treatment. The true effect of selecting a treatment is a combination of biological effects, variations in compliance or adherence, and other patient characteristics that influence efficacy. Only by retaining all patients intended to receive a given treatment in their original treatment group can researchers and clinicians obtain an unbiased estimate of the effect of selecting one treatment over another.