Many individuals have studied the phenomena of the obsolescence of information. In general, the principle is that an information document--let's say a journal article--has a "life cycle." Soon after publication a journal article is used relatively frequently, but as time goes by and the paper is superceeded by newer research and ideas use of the article declines.
Now, use can be measured a number of different ways (e.g., requests for the document via interlibrary loan or document delivery, citations to the publication, or number of times a book is circulated).
In addition, the rate and pattern at which usage drops of has been shown to vary by discipline. Generally speaking, documents in the physical sciences receive most of their use early in their "lives" and then drop off rather steeply, whereas documents in the humanities tend to be used on a more consistent basis and have a less steep drop off curve.
The term "half-life" comes from Burton, R. E., and R. W. Kebler. 1960. The "half-life" of some scientific and technical literatures. American documentation 11: 18-22. It makes an analogy to the radioactive decay of nuclear particles and assumes an exponential curve.
My ascii figure below illustrates the general principle.
9x
^ 8x *
| 7x * *
Increasing 6x *
usage 5x *
| 4x *
| 3x *
2x *
1x * *
0x * *
0x 1x 2x 3x 4x 5x 6x 7x 8x 9x
-- Passage of time -->
There are many articles on this concept. A fairly recent one to begin with might be:
Gupta, Usha. 1990. Obsolescence of physics literature: exponential decrease of the density of citations to Physical Review articles with age. Journal of the America Society for Information Science 41(4): 282-287.
ISI's Journal Citation Reports (1997) defines "cited half-life" as "the number of journal publication years going back from the current year which account for 50% of the total citations received by the cited journal in the current year."
Markwell and Brooks report that a study of web links in a corpus of online biology courses have a half-life of 55 months (http://www-class.unl.edu/biochem/url/broken_links.html.
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