4. The method of claim 1 , further comprising, for each one of the plurality of corpora, determining, by a processing device, a normalization factor based on a subset of the inverse document frequency values associated with the one of the plurality of corpora and a set of reference inverse document frequency values.
4. The method of claim 1 , further comprising, for each one of the plurality of corpora, determining, by a processing device, a normalization factor based on a subset of the inverse document frequency values associated with the one of the plurality of corpora and a set of reference inverse document frequency values. 5. The method of claim 4 , further comprising: for each term in the subset of the terms that also appear in the document, determining, by a processing device, the plurality of normalized inverse document frequency values each: (a) associated with a respective one of the plurality of corpora and (b) based on: (i) the normalization factor that was determined for the respective one of the plurality of corpora and (ii) the inverse document frequency value determined for the term in the subset of the terms and associated with the respective one of the plurality of corpora.