Since demand is a basic factor in selection you have to develop a system for ascertaining the demand- Demand includes both expressed wants and unexpressed wants and unexpressed needs of readers. Similarly, needs of both actual and potential readers should be taken into account.
Perhaps, the following sources can be made use of in ascertaining demand:
• statistics relating to books circulated from, and consulted in the library;
• suggestions from library staff working in the service departments;
• suggestions from readers;.
• indents from subject experts or departments;
• findings of user need survey conducted;
• syllabi and prospectuses of courses of studies; and
• profiles of researchers.
Demands have to be assessed in terms of their volume, value and variety. The final rating of the demand should COMIM with the selection policy of the library. Each library ought to have its own broad selection policy. Some libraries even have a written statement of this policy. In either case, the policy is the basic guideline in selection of documents.