A. Singhal, C. Buckley and M. Mitra. Pivoted Document Length Normalization. In Proceedings of the 19th Annual International ACM SIGIR Conference on Research and Development in Information Retrieval. Pages 21 - 29. Zurich, Switzerland. 1996,
Singhal et. al. give the following two reasons for the need for the term frequency normalisation:
The same term usually occurs repeatedly in long documents.
A long document has usually a large size of vocabulary.
As a consequence, an information retrieval system without term frequency normalisation produces biased results and favours long documents.