  A complete and critical survey is presented of all half-life, decay-energy
and branching-ratio measurements related to 20 superallowed decays; no
measurements are ignored, though some are rejected for cause and others
updated. A new calculation of the statistical rate function is described and
experimental ft 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 new
limits on any possible scalar interactions or right-hand currents. The average
Ft value obtained from the survey, when combined with the muon lifetime, yields
the CKM matrix element Vud = 0.9738(4); and the unitarity test on the top row
of the matrix becomes |Vud|^2 + |Vus|^2 + |Vub|^2 = 0.9966(14) using the PDG's
currently recommended values for Vus and Vub. We discuss the priorities for
future theoretical and experimental work with the goal of making the CKM
unitarity test more definitive.
