  This decade has seen a great deal of progress in the development of
information retrieval systems. Unfortunately, we still lack a systematic
understanding of the behavior of the systems and their relationship with
documents. In this paper we present a completely new approach towards the
understanding of the information retrieval systems. Recently, it has been
observed that retrieval systems in TREC 6 show some remarkable patterns in
retrieving relevant documents. Based on the TREC 6 observations, we introduce a
geometric linear model of information retrieval systems. We then apply the
model to predict the number of relevant documents by the retrieval systems. The
model is also scalable to a much larger data set. Although the model is
developed based on the TREC 6 routing test data, I believe it can be readily
applicable to other information retrieval systems. In Appendix, we explained a
simple and efficient way of making a better system from the existing systems.
