Table 3 Kendall's tau (rank correlation) comparing the most significant exact p-values (the reference) to the Karlin's approximations. The column "all" gives the result for all sequences while the R_{
i
}give the results for a certain range of sequence lengths: smaller than 100 for R1, between 100 and 200 for R2, between 200 and 500 for R3, between 500 and 1 000 for R4, between 1 000 and 2 000 for R5 and greater than 2 000 for R6.