Watterson estimator

In Population Genetics the Watterson Estimator is a method for estimating $$\theta = 4N_e\mu$$. Where $$N_e$$ is the effective population size and $$\mu$$ is the mutation rate of the population of interest. The assumptions made are that there are n samples of individuals from the population of interest, there are an infinitely many alleles possible, and that $$n << N_e$$.

The estimate of $$\theta$$ is

$$ \theta = { K \over a_n }. $$

Where K is the number of segregating sites in the sample and

$$ a_n = \sum^{n-1}_{i=1} {1 \over i}. $$

This estimate is based on Coalescent theory.