Two other points to keep in mind:
1. CD-ROMs tend to be at least somewhat platform dependent, with a
dependence either upon a specific operating system (e.g., Windows) or
a specific programming environment (e.g., Java). It's quite easy to
make an on-line resource platform independent.
2. Subscription prices. When your price structure is so high that a
physical copy of your product is about the same price as a yearly
subscription, the cost benefit analysis doesn't come out in favor of
the online version. If a librarian is looking at a $500 per year
subscription that allows only one user access to a resource, versus a
$500 per volume cost for a print edition, unless the updates are
very, very compelling, that librarian is going to by the print volume
(which will last much longer). In the end, I think the CD-ROM may be
a slightly MORE stable medium for the librarian who must pay for the
resource: when the subscription runs out at the end of the year, the
resource is effectively de-accessioned. A CD-ROM will usually last at
least 10 years (in my experience), if it's stamped (not burned).
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