  We show in microwave experiments and random matrix calculations that in
samples with a large number of channels the statistics of transmission for
different incident channels relative to the average transmission is determined
by a single parameter, the participation number of the eigenvalues of the
transmission matrix, M. Its inverse, M-1, is equal to the variance of relative
total transmission of the sample, while the contrast in maximal focusing is
equal to M. The distribution of relative total transmission changes from
Gaussian to negative exponential over the range in which M-1 changes from 0 to
1. This provides a framework for transmission and imaging in single samples.
