  The Vud element of the Cabibbo-Kobayashi-Maskawa matrix can be determined
from several different experimental approaches: either 0+-to-0+ superallowed
nuclear beta decays, neutron decay, nuclear mirror decays, or pion beta decay.
Currently all give consistent results but, because the nuclear superallowed
value has an uncertainty at least a factor of seven less than all other
results, it dominates the result. A new survey of world superallowed-decay data
establishes the Ft values of 14 separate superallowed transitions to a
precision of order 0.1% or better; and all 14 are statistically consistent with
one another. This very robust data set yields the result Vud = 0.97417(21), the
value we recommend.
