TABLE OF CONTENTS In engineering practice we are often interested in the average behavior of measurements on random processes. The goal of this chapter is to link the two distinct types of averages that are used long-term time averages taken by calculations on an actual physical realization of a random process and averages calculated theoretically by probabilistic calculus at some given instant of time, averages that are called expectations. As we shall see, both computations often (but by no means always) give the same answer. Such results are called laws of large numbers or ergodic theorems.
At first glance from a conceptual point of view, it seems unlikely that long-term time averages and instantaneous probabilistic averages would be the same. If we take a long-term time average of a particular realization of the random process, say { X( t, ? 0); t ? 
}, we are averaging for a particular ? which we cannot know or choose; we do not use probability in any way and we are ignoring what happens with other values of ?. Here the averages are computed by summing the sequence or integrating the waveform over t while ? 0 stays fixed. If, on the other hand, we take an instantaneous probabilistic average, say at the time t 0, we are taking a probabilistic average and summing or integrating over ? for the random variable X( t 0, ?).
