Watterson estimator
= Watterson estimator
{c}
{title2=$\widehat\theta_W=\frac{S}{\sum_{i=1}^{n-1}1/i}$}
For a neutral sample of size $n\ge2$ under the <infinite sites mutation model>, the Watterson estimator is $\widehat\theta_W=S/a_{n-1}$, 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>.