Autocorrelation between values that are at a constant distance apart in time or space. Suppose x1, x2,…, xn is an ordered sequence of observations. The serial correlation at lag k is rk, given bywhereIn practice, a less complicated formula is routinely used. This formula, which involves x̄, the overall mean, isThe term ‘serial correlation’ was coined by Yule in a 1926 paper.