automated classification systems, but also to filters that use only human review.  Given

the approximately two billion pages that exist on the Web, the 1.5 million new pages that

are added daily, and the rate at which content on existing pages changes, if a filtering

company blocks only those Web pages that have been reviewed by humans, it will be

impossible, as a practical matter, to avoid vast amounts of underblocking.  Techniques

used by human reviewers such as blocking at the IP address level, domain name level, or

directory level reduce the rates of underblocking, but necessarily increase the rates of

overblocking, as discussed above. 

To use a simple example, it would be easy to design a filter intended to block

sexually explicit speech that completely avoids overblocking.  Such a filter would have

only a single sexually explicit Web site on its control list, which could be re reviewed

daily to ensure that its content does not change.  While there would be no overblocking

problem with such a filter, such a filter would have a severe underblocking problem, as it

would fail to block all the sexually explicit speech on the Web other than the one site on

its control list.  Similarly, it would also be easy to design a filter intended to block

sexually explicit speech that completely avoids underblocking.  Such a filter would

operate by permitting users to view only a single Web site, e.g., the Sesame Street Web

site.  While there would be no underblocking problem with such a filter, it would have a

severe overblocking problem, as it would block access to millions of non sexually

explicit sites on the Web other than the Sesame Street site.  

While it is thus quite simple to design a filter that does not overblock, and equally

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