手法を比較

	Citation Half-Life and Literature Obsolescence	Immediacy Index
分野	計量書誌学	計量書誌学
系統	Process / pipeline	Process / pipeline
提唱年	1960	1972
提唱者	Robert E. Burton & R. W. Kebler; Maurice B. Line	Eugene Garfield (ISI / Journal Citation Reports)
種類	Citation-aging pipeline	Same-year citation-rate pipeline
原典	Burton, R. E., & Kebler, R. W. (1960). The "half-life" of some scientific and technical literatures. American Documentation, 11(1), 18-22.	Garfield, E. (1972). Citation analysis as a tool in journal evaluation. Science, 178(4060), 471-479.
別名	Literature Half-Life, Literature Obsolescence, Citation Aging, Literature Decay Rate	Journal Immediacy Index, Same-Year Citation Rate, Current Citation Rate, Year-of-Publication Citation Index
関連	3	3
概要	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 Immediacy Index measures how quickly a journal's articles are cited by counting the citations they receive in the very year they are published. Eugene Garfield, who created the Science Citation Index and the impact factor, introduced the immediacy index as part of the Institute for Scientific Information's journal-evaluation methodology and described it in his landmark 1972 Science paper on citation analysis as a tool in journal evaluation. While the impact factor averages citations over a two-year window, the immediacy index uses a zero-year window: it divides the citations made in a given year to that year's articles by the number of citable items published that year. A high immediacy index means a journal's work is picked up almost immediately, the signature of a fast-moving, frontier field or of journals that publish hot, rapidly cited material. It is a measure of citation speed rather than of total impact.