To compare the similarity between symbolic sequences, we plotted the rank number of each n-tuple sequence in the first sequence against that of the second sequence ( Figure 1B ). If two sequences are similar in their rank order of n-tuples, the scattered points will be located near the diagonal line (e.g., comparison between healthy subjects) [17] . Therefore, the average deviation of these scattered points away from the diagonal line is a measure of the dissimilarity index between these two sequences.