Errata

This section provides formalas for two key information retrieval priciples, recall and precision. The formula for precision is incorrect. Precision of search results is the number of relevant documents retreived by the query out of all the documents retrieved by the query. In other words, of all the documents my search retreived, how many are actually relevant. Hence, the denominator of the formula provided in te book needs correction.

Precision= # relevant documents retrieved/# total documents retreived

Testing for how many relevant docs were retrieved out of all the docs in the collection (as your current precision formula implies) is not very useful since it doesn't tell you whether more relevant docs were there to be had or not. Example, if get 2 relevant docs out of the whole collection, maybe there were only two, maybe there were 8 more. Who knows? However, if only 2 out of 10 docs retrieval are relevant (by the correct Precision formula), that tells me something about the accuracy or "precision" of my system's matching capability.

Also, if I know that there are 10 relevant docs in the collection and I get 8 relevant docs in my results (by the Recall formula), that tells me something about the breadth of my system's matching capability.