A complete and critical survey is presented of all half-life, decay-energy and branching-ratio measurements related to 20 superallowed 0 + → 0 + decays; no measurements are ignored, though some are rejected for cause and others updated. A new calculation of the statistical rate function f is described and experimental f t values determined. The associated theoretical corrections needed to convert these results into Ft values are discussed, and careful attention is paid to the origin and magnitude of their uncertainties. As an exacting confirmation of the conserved vector current hypothesis, the Ft values are seen to be constant to 3 parts in 10 4 . These data are also used to set a new limit on any possible scalar interaction: C S /C V = −(0.00005 ± 0.00130). The average Ft value obtained from the survey, when combined with the muon liftime, yields the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa matrix, V ud = 0.9738 ± 0.0004; and the unitarity test on the top row of the matrix becomes |V ud | 2 + |V us | 2 + |V ub | 2 = 0.9966 ± 0.0014 using the Particle Data Group's currently recommended values for V us and V ub . We also express this result in terms of the possible existence of right-hand currents. Finally, we discuss the priorities for future theoretical and experimental work with the goal of making the CKM unitarity test more definitive.

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