ABSTRACT
Analysis of covariance (Ancova) can reduce variability due to individual differences, thereby yielding narrower confidence intervals and greater power. Prime opportunities arise with experiments that compare effectiveness of methods to produce some change: change in health, behavior, attitude, or skill. The change score, after—before, may seem the natural measure; this change score seems to adjust for individual differences in the before score. But change scores turn out to be full of pitfalls. Ancova not only avoids these pitfalls, but gives a superior analysis.
