The six multicenter studies from which the data were drawn included: the Multicenter Post Infarction Program (MPIP), a longitudinal, observational study of 867 patients; the Multicenter Diltiazem Post Infarction Trial (MDPIT), a double-blind, randomized, placebo-controlled trial of 2,466 patients; the Cardiac Arrhythmia Pilot Study (CAPS), a double-blind, randomized, placebo-controlled trial of 502 patients; the Cardiac Arrhythmia Suppression Trial (CAST), a double-blind, randomized, placebo-controlled trial of 4,200 patients; Electrophysiologic Studies Versus Electrocardiographic Monitoring (ESVEM), a comparison of two methods for evaluating antiarrhythmic drug efficacy in 350 patients; and the Cardiac Rate/Rhythm in Normal Adults, a cross-sectional observational study of 250 subjects.