In population genetics, the Watterson estimator is a method for describing the genetic diversity in a population. It was developed by Margaret Wu and G. A. Watterson in the 1970s. It is estimated by counting the number of polymorphic sites. It is a measure of the "population mutation rate" from the observed nucleotide diversity of a population. , where is the effective population size and is the per-generation mutation rate of the population of interest. The assumptions made are that there is a sample of haploid individuals from the population of interest with effective size , that , and that there are infinitely many sites capable of varying . Because the number of segregating sites counted will increase with the number of sequences looked at, the correction factor is used.