Some indexes cover a broad range of subjects while other indexes are devoted to a single discipline.
Indexes that provide summaries of the articles listed in them are called abstracts. Examples of indexes with abstracts are Chemical Abstracts and Psychological Abstracts . Print volumes provide references to the literature published during a particular period, often in monthly and annual volumes. Database indexes cover the literature spanning several years or more. It is important to know if the index timespan is adequate for any particular search.