McColvin's Principles:

L.R McColvin advanced his Demand and Supply Theory of Book Selection in 1925. He states, "Books in themselves are nothing. They have no more meaning than the white paper upon which they are printed, until they are made serviceable by demand. The more closely book selection is related to demand, the greater is the resultant and possible service". The term "supply" refers to the availability of  reading material in all its varieties. "Demand", on the other hand, means expressed and unexpressed informational needs of the users. This theory advocates the selection of only those documents which are demanded by the users for their informational needs. Demand for documents, however, should be differentiated according to its volume, value and variety. Whereas the development of the collection should be need-based, it should at the same time be seen that we develop a balanced collection. No document of value should be left out even if demand does not exist for it. Such documents are likely to be in demand in future. It should also be seen that no section of the users is over-emphasised or neglected.