plaintiffs' expert witness, Dr. Geoffrey Nunberg, that no
software exists that can automatically distinguish visual
depictions that are obscene, child pornography, or harmful to
minors, from those that are not. Nor can software, through
keyword analysis or more sophisticated techniques, consistently
distinguish web pages that contain such content from web pages
that do not.


In light of the absence of any automated method of
classifying Web pages, filtering companies are left with the
Sisyphean task of using human review to identify, from among the
approximately two billion web pages that exist, the 1.5 million
new pages that are created daily, and the many thousands of pages
whose content changes from day to day, those particular web pages
to be blocked. To cope with the Web's extraordinary size, rate
of growth, and rate of change, filtering companies that rely
solely on human review to block access to material falling within
their category definitions must use a variety of techniques that
will necessarily introduce substantial amounts of overblocking.
These techniques include blocking every page of a Web site that
contains only some content falling within the filtering
companies' category definitions, blocking every Web site that
shares an IP-address with a Web site whose content falls within
the category definitions, blocking "loophole sites," such as
anonymizers, cache sites, and translation sites, and allocating
staff resources to reviewing content of uncategorized pages
rather than re-reviewing pages, domain names, or IP-addresses
that have been already categorized to determine whether their
content has changed. While a filtering company could choose not