For a discrete process with known mean and variance for which we observe n observations \{X_1,\,X_2,\,\ldots,\,X_n\}, an estimate of the autocorrelation may be obtained as .
\hat{R}(k)=\frac{1}{(n-k) \sigma^2} \sum_{t=1}^{n-k} (X_t-\mu)(X_{t+k}-\mu) .