URLs: Mean time of survival?

[Heinrich C. Kuhn]

Dear Web4Lib-ers,

   in a paper I'm writing I'll have to add a line con-
cerning the tendency of URLs to change and to become
obsolete, the tendency of electronic documents to
wander from one URL to annother and their tendency to
vanish. Experience seems to show, that the chances that
a certain document will change its location or become
unavailable are to some degree connected to the type
of the document (e.g. announcements for conferences are
on the mean less longliving than are collections of
conference papers) and to the status of the persons
who produced the documents (e.g. students' homepages 
being less likely to survive for 2 years than libraries'
homepages [:-)] ), and perhaps even to the subject of
the document.

   Does anyone know about any statistical information
on the "mean time of survival" of URLs and electronic
documents on the web in relation to a) type of ressource,
b) status of author, c) subject?

Thanks a lot for any answer! (Proper acknowledgements 
will of course be made ...).

TIA

Heinrich C. Kuhn
+---------------------------------------------------------
!   Dr. Heinrich C. Kuhn   (coordinator libraries &c.)
!   Max-Planck-Gesellschaft / Generalverwaltung VIIIb3
!   Postfach 10 10 62 /  D-80084 Muenchen
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