Auto correlation: The correlation of the internal observations in the time series, generally expressed as a function of the time lag between the observations. It is also used for the correlations between points different distances apart in the set of spatial data (that is spatial autocorrelation). The autocorrelation at lag k, γ(k), is de?ned mathematically as follows
here Xt ; t 0; 1; 2; ... stand for the values of the series and µ is the mean of series.