For each document in the top 20 hits on any service, we judged the document to be relevant or nonrelevant. Then, using this master list of relevant documents, we went back and calculated the precision and recall for each service using result lists of various sizes. Finally, we made a graph for each service based on different pairs of precision and recall. The graph shows the precision at different levels of recall as the result-set for each service grows larger and larger. The best result would be a line all the way across the very top of the graph.
Nobody's perfect, so none of the services achieve that.
Note that Excite does best on both those counts. If a service stays low on the graph, then that means that the result list is cluttered with a lot of irrelevant hits. If a service doesn't extend very far to the right, then that means that the service never brings back very many good hits, no matter how long the result list gets. The bottom line is that Excite does the best job of bringing back relevant documents in your result list. Lots of services can claim to have large databases, but it's much harder to preserve search accuracy as your database grows.