Numerical comparison of several approximations of the word count distribution in random sequences

The exact distribution of word counts in random sequences and several appro ximations have been proposed in the past few years. The exact distribution has no theoretical limit but may require prohibitive computation time. On t he other hand, approximate distributions can be rapidly calculated but, in practice, are only accurate under specific conditions. After making a surve y of these distributions, we compare them according to both their accuracy and computational cost. Rules are suggested for choosing between Gaussian a pproximations, compound Poisson approximation, and exact distribution. This work is illustrated with the detection of exceptional words in the phage L ambda genome.