CD-ROM is making rapid inroads into the union catalog field. Wisconsin has decided to publish its 3.1 million title, 12.5 million holdings data base on two CD-ROM discs. The data base will be indexed by author, title, subject, LCCN, ISBN/ISSN, and other fields. Maine has reached the same decision as has MOBAC, a regional system in California. While the cost of the medium compares favorably with microform, the cost of the equipment to access the data is up to twenty times higher. For the extra cost, one gets online searching instead of serial access inherent in microform systems.
A union catalog on CD-ROM does not pose the same performance issues that a patron access catalog does. A patron access catalog involves a large number of subject searches, including Boolean searches. Some search strategies can adversely affect response time. Not only is a union catalog consulted less frequently than a library's own patron access catalog, it is also used primarily for known item searches that involve fewer index accesses.