The scatter of documents over databases in different subject domains: How many databases are needed?

The distribution of bibliographic records in on-line bibliographic databases is examined using 14 different search topics. These topics were searched using the DIALOG database host, and using as many suitable databases as possible. The presence of duplicate records in the searches was taken into consideration in the analysis, and the problem with lexical ambiguity in at least one search topic is discussed. The study answers questions such as how many databases are needed in a multifile search for particular topics, and what coverage will be achieved using a certain number of databases. The distribution of the percentages of records retrieved over a number of databases for 13 of the 14 search topics roughly fell into three groups: (1) high concentration of records in one database with about 80% coverage in five to eight databases; (2) moderate concentration in one database with about 80% coverage in seven to 10 databases; and (3) low concentration in one database with about 80% coverage in 16 to 19 databases. The study does conform with earlier results, but shows that the number of databases needed for searches with varying complexities of search strategies, is much more topic dependent than previous studies would indicate.