Analysis of monograph obsolescence at two levels of an interlibrary loan network

Requests for monographs generated within an interlibrary loan network are analyzed for half-life statistics. It is suggested that demand represents use of the literature more completely than satisfied requests or circulation statistics. Demand in this study is characterized as either regional demand or statewide demand and is related to the level of the network where final processing of the request occurs. A negative exponential distribution is found to adequately characterize both levels of demand as a function of publication date for four subject categories. Corrected demand data is obtained by removing the growth rate of most of the available literature represented by American book publisher output. Based on over 10,000 interlibrary loan requests, negative exponential distributions describe the raw data as well as the corrected data. A shorter half-life was found for regional library demand (10.47 yr) than that found for statewide library demand (15.75 yr). Applying the correction factor to reflect the growth rate of the available literature tends to increase the half-life when compared to the raw data, and changes the ordering of subject classes with respect to ascending half-lives.