A major fear of library users is that of missing vital information through lack of access to every item. This is one of the most commonly expressed fears resulting from the information explosion, yet it is quite irrational because total access never has been achievable. The production of a fully comprehensive list of references to a topic is impossible, and the further the search is extended towards comprehensiveness, the higher the production costs rise and the lower the probability that the retrieved references will be of significant quality.