 Watterson estimator

For a neutral sample of size $n\ge 2$ under the infinite sites mutation model, the Watterson estimator is ${\hat{\theta }}_W=S/a_{n-1}$, where $a_{n-1}={\sum }_{i=1}^{n-1}1/i$. Since the coalescent has $ES=\theta a_{n-1}$, this estimator is unbiased. It matches the observed segregating-site count to its expected value, but need not maximize the exact segregating-site likelihood under the neutral coalescent.

The Watterson estimator is a statistical method used in population genetics to estimate the theta (\( \theta \)) parameter, which represents the population mutation rate per generation. The estimator is based on the number of polymorphic sites in a sample of DNA sequences and is particularly useful for inferring levels of genetic diversity within a population.