THE USE OF OLDER LITERATURE AND ITS OBSOLESCENCE

Terminology and parameters for describing the relation between figures for the use of library literature and for its age are discussed. Unless a correction for growth is applied, half‐life fails a simple test of suitability, as do Brookes's ageing and utility factors. Results based on cumulated data have other important disadvantages. Relative ‘use‐per‐item’ figures are satisfactory. The need to relate terminology more closely to the facts they represent is stressed: item‐consultation decay rate, and 50% consultation probability age are suggested and defined. In the context of citation studies, item‐citation decay rate and 50% citation probability age are equivalent and avoid similar objections. In data from NRLSI, item‐consultation decay rates are only approximately exponential. Different rates are demonstrated for updating and basic searches, for academic and other types of readers, and for types of literature. For some historical searches negative decay rates were found in the NRLSI and BML.