Abstract: A method by which a sponsor (24) audits convention registrants attending an event who occupy rooms at hotels (26a-26D) that the registrant does not book through the sponsor's housing provider (32). Before the event, the sponsor (24) arranges a commitment from hotels (26A-26D) for a block of rooms during the event. The auditing method includes producing an out-of-block list (64) that identifies registrants who do not appear in an in-block list (56). The out-of-block list (64) is stored on a housing-audit computer (52) from which the hotels (26A-26D) download copies. The hotels (26A-26D) in producing a match list (82) find matches between their guest list (76) and the downloaded copy of the out-of-block list (64). After validating and accepting matches between the lists (64, 76), the hotels (26A-26D) upload the match list (84) to the housing-audit computer (52) for use in producing audit reports (88) supplied to the sponsor (24).