The principle of parallelism in the design of studies to estimate treatment effects.

An effect is a function of a cause as well as of 4 other factors: recipient, setting, time, and outcome variable. The principle of parallelism states that if a design option exists for any 1 of these 4 factors, a parallel option exists for each of the others. For example, effects are often estimated by drawing a comparison across recipients who receive different treatments. The principle of parallelism implies that an effect can also be estimated by drawing a comparison across settings, times, or outcome variables. Typologies of methodological options are derived from the principle of parallelism. The typologies can help researchers recognize a broader set of options than they would otherwise and thereby improve the quality of research designs.