Citation Half-Life and Literature Obsolescence

Citation half-life measures how quickly a body of literature ages by finding the median age of the documents being cited. Borrowing the metaphor of radioactive decay, Robert Burton and R. W. Kebler proposed in 1960 that scientific literatures grow obsolete at characteristic rates, and that the half-life, the time within which half of the currently used references were published, summarizes this aging. A short half-life means a field draws mostly on recent work and old papers fall out of use quickly; a long half-life means older literature stays relevant. Maurice Line's 1970 critique drew a crucial distinction between apparent obsolescence, the decline in citations to older work, and real obsolescence, separating the genuine decline in a paper's usefulness from the statistical artifact created by the rapid growth of the literature, which makes recent years appear disproportionately important simply because there are more of them.