We analyze the kinematic conditions under which the scattering of equivalent
massive spin-1 vector bosons factorizes out of the complete process. In
practice, we derive the conditions for the validity of the effective W
approximation, proposed long ago but never established on a firm basis. We also
present a parametric estimate of the corrections to the approximation and
explicitly check its validity in two examples.Comment: 36 pages, 14 figures, references adde