In an extended review ofSequential Medical Trialsby P. Armitage, the statistical principles which should govern the analysis of experimental observations, and the planning of experiments, are discussed. It is suggested that the operating-characteristic concepts of the Neyman-Pearson theory of tests are inappropriate to the analysis and interpretation of experimental data; the likelihood principle should be followed instead. The planning of medical trials under an ethical injunction against unnecessary continuance of inferior treatments is studied in some detail. The propriety of such trials is considered.