Sample autocorrelation statistic for original eight time series and their shuffle surrogates. For each of the eight time series, surrogates are created by randomly permuting the original time series. The sample autocorrelation function up to a lag of 6 is then shown for the surrogates, and that for the original series is shown in a dark line. The thin red line shows the median of the surrogates. The top left plot shows a time series that is indistinguishable from white noise. The other time series show serial correlation.