The expectations or ensemble average of a random process x(t) are averages "across the process". The DC value of x(t) is defined by the time average The other time average of particular interest is the autocorrelation function Rx(t , T) defined in terms of the sample function x(t) observed over the interval - T ≤ t ≤ T. Following equation, we may formally define the time-averaged autocorrelation function of a sample function x(t) as follows : This second time-average should also be viewed as a random variable with a mean and variance of its own. In a manner similar to ergodicity of the mean, we say that the process x(t) is ergodic in the autocorrelation function if the following two limiting conditions are satisfied : .