| Clinical outcomes depend on treatment AND other
factors: underlying severity of illness, comorbid conditions, and
other prognostic factors that may be unknown. Randomization ensures
treatment is only factor affecting difference in outcome between two groups
in the study.
Factors affecting clinical outcome also influence investigators decisions to enroll in the treatment group. What result does the researcher want? Nonrandomized studies tend to show larger (usually false positive) treatment effects than do randomized trials. |