Many server selection methods suitable for distributed information retrieval applications rely, in the absence of cooperation, on the availability of unbiased samples of documents from the constituent collections. We describe a number of sampling methods which depend only on the normal query-response mechanism of the applicable search facilities. We evaluate these methods on a number of collections typical of a personal metasearch application. Results demonstrate that biases exist for all methods, particularly toward longer documents, and that in some cases these biases can be reduced but not eliminated by choice of parameters.We also introduce a new sampling technique, "multiple queries", which produces samples of similar quality to the best current techniques but with significantly reduced cost.
['Paul Thomas', 'David Hawking']
Evaluating sampling methods for uncooperative collections