The hypergeometric model assumes that the document is a sample, and the population is from the collection. It is called DLH, in our DivergenceFromRandomness terminology. The DLH model is a generalistion of the hypergeometric model in a binomial case.
This model is parameter-free and includes an implicit TermFrequencyNormalisation component.
The formula of the DLH model can be found in FormulasOfDFRModels.