evidence not only that filtering programs bar access to a
substantial amount of speech on the Internet that is clearly
constitutionally protected for adults and minors, but also that
these programs are intrinsically unable to block only illegal
Internet content while simultaneously allowing access to all
protected speech.
As our extensive findings of fact reflect, the plaintiffs
demonstrated that thousands of Web pages containing protected
speech are wrongly blocked by the four leading filtering
programs, and these pages represent only a fraction of Web pages
wrongly blocked by the programs. The plaintiffs' evidence
explained that the problems faced by the manufacturers and
vendors of filtering software are legion. The Web is extremely
dynamic, with an estimated 1.5 million new pages added every day
and the contents of existing Web pages changing very rapidly.
The category lists maintained by the blocking programs are
considered to be proprietary information, and hence are
unavailable to customers or the general public for review, so
that public libraries that select categories when implementing
filtering software do not really know what they are blocking.


There are many reasons why filtering software suffers from
extensive over- and underblocking, which we will explain below in
great detail. They center on the limitations on filtering
companies' ability to: (1) accurately collect Web pages that
potentially fall into a blocked category (e.g., pornography); (2)
review and categorize Web pages that they have collected; and (3)
engage in regular re-review of Web pages that they have
previously reviewed. These failures spring from constraints on