The expected value availability is simply the average value of
the function over all possible values of the variable. When we discuss a
system’s steady state availability, we are referring, on the other hand, to
the behavior of an ensemble of equipments. If we had a large number of
equipments that have been operating for some time, then at any particular time
we would expect the number of equipments that are in state 0 (available) to be
NP_{0} . Thus, the ratio of the number of equipments available to the
total number of equipments is simply NP_{0}/N =
P_{0}, where N = total number of
equipments and P_{0} is
fraction of total equipment (N) in state 0
(available).