Measuring recall

Recall, the proportion of the relevant documents that is retrieved, is a key indicator of the performance of an information retrieval system. With large information systems, like the World Wide Web on the Internet, recall is almost impossible to measure or estimate by all standard techniques. A proposal 'needle hiding' is made as a technique to estimate recall. It is also shown that ranking by relative recall does not have to be isomorphic to ranking by recall; hence the use of relative recall for comparative evaluation might not be entirely sound.