IP address associated with a particular Web site is blocked under a particular category

and the Web site goes out of existence, then the IP address likely would be reassigned to

a different Web site, either by an Internet service provider or by a registration

organization, such as the American Registry for Internet Numbers, 

see

http://www.arin.net.  In that case, the site that received the reassigned IP address would

likely be miscategorized.   Because filtering companies do not engage in systematic re 

review of their category lists, such a site would likely remain miscategorized unless

someone submitted it to the filtering company for re review, increasing the incidence of

over  and underblocking.  

This failure to re review Web pages primarily increases a filtering company's rate

of overblocking.  However, if a filtering company does not re review Web pages after it

determines that they do not fall into any of its blocking categories, then that would result

in underblocking (because, for example, a page might add sexually explicit content).   

3.  The Inherent Tradeoff Between Overblocking and Underblocking 

There is an inherent tradeoff between any filter's rate of overblocking (which

information scientists also call  precision ) and its rate of underblocking (which is also

referred to as  recall ).  The rate of overblocking or precision is measured by the

proportion of the things a classification system assigns to a certain category that are

appropriately classified.  The plaintiffs' expert, Dr. Nunberg, provided the hypothetical

example of a classification system that is asked to pick out pictures of dogs from a

database consisting of 1000 pictures of animals, of which 80 were actually dogs.  If it

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