Autocorrelation refers to the correlation of a time series .It is also called “lagged correlation” which refers to the correlation between members of a series of numbers arranged in time. Three tools for assessing the autocorrelation of time series are the time series plot, the lagged scatterplot and autocorrelation function.
An important guide to the persistence in a time series is given by the series of quantities called the sample autocorrelation coefficient, which measure the correlation between observations at different times. The correlation of observations separated by