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	Citation Half-Life and Literature Obsolescence	Price Index (Citation Recency)
Field	Bibliometrics	Bibliometrics
Family	Process / pipeline	Process / pipeline
Year of origin	1960	1970
Originator	Robert E. Burton & R. W. Kebler; Maurice B. Line	Derek J. de Solla Price
Type	Citation-aging pipeline	Reference-recency pipeline
Seminal source	Burton, R. E., & Kebler, R. W. (1960). The "half-life" of some scientific and technical literatures. American Documentation, 11(1), 18-22.	Price, D. J. de Solla (1965). Networks of scientific papers. Science, 149(3683), 510-515.
Aliases	Literature Half-Life, Literature Obsolescence, Citation Aging, Literature Decay Rate	Price's Index, Citation Recency Index, Share of Recent References, Research-Front Index
Related	3	3
Summary	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.	The Price Index measures how strongly a field draws on recent literature by computing the percentage of its references that point to work published within the last few years. Derek de Solla Price, the founder of modern scientometrics, observed that the sciences differ sharply in how immediate their referencing is: hard sciences cite a tight cloud of recent papers at the research front, while humanities scholarship cites a long, even spread of older works. In his 1965 Science paper on the networks of scientific papers he documented this concentration of references on recent years, and in his 1970 essay distinguishing hard science, soft science, technology, and nonscience he formalized the index that now bears his name. Defined as the share of references no older than about five years, the Price Index is a citing-side companion to obsolescence measures: where the cited half-life looks at how the literature ages, the Price Index looks at how recency-focused the citing behavior is.