  A length dependence of the effective mobility in the form of a power law, B ~
L^(1-1/alpha) is observed in dispersive transport in amorphous substances, with
0 < \alpha < 1. We deduce this behavior as a simple consequence of the
statistical theory of extreme events. We derive various quantities related to
the largest value in samples of n trials, for the exponential and power-law
probability densities of the individual events.
