  The transmission through a stack of identical slabs that are separated by
gaps with random widths is usually treated by calculating the average of the
logarithm of the transmission probability. We show how to calculate the average
of the transmission probability itself with the aid of a recurrence relation
and derive analytical upper and lower bounds. The upper bound, when used as an
approximation for the transmission probability, is unreasonably good and we
conjecture that it is asymptotically exact.
