The DivergenceFromRandomness models apply the normalisation 2 for the TermFrequencyNormalisation.

The normalisation 2 assumes a decreasing term frequency density of DocumentLength. The formula of the normalisation 2 is as follows:

tfn = tf * log(1 + c*(sl/dl))

where tfn and tf are the normalised and original term frequency of the query term in the document. sl is the average **document length** in the whole collection. dl is the **Document Length**. The parameter c can be set automatically, as described by He and Ounis 'Term Frequency Normalisation Tuning for BM25 and DFR model', in Proceedings of ECIR'05, 2005.