If a manual is not available or is incomplete, the convenient but less exhaustive alternatives are the help screens in CD-ROM systems. These provide summary statements about the database and time periods covered. Other excellent sources are the data sheets from online vendors that describe the databases available through them. Because of per minute costs, these can be expensive. Several other print database directories give additional guidance on database coverage and are available in larger libraries.3 Scope and Coverage After selecting the likely databases to use, determine the scope and depth of coverage.
Periodicals are generally the largest component of scientific bibliographic databases.
In general, news items, letters, book reviews, and shorter communications are commonly excluded. Dissertations, patents, and books are covered unevenly among databases. Databases also vary in the degree of currency of its records compared with the dates of the source publications. Because of the labor-intensive nature of indexing, it is not unusual for the online database to contain a reference to a source publication 3 months behind the date of the original. This potentially long delay should alert the user not to rely solely on one or even two databases for literature coverage.