that, while possibly harmful to minors, are neither obscene nor
child pornography. See supra, Subsection II.E.7. Even the
defendants' own expert, after analyzing filtering products'
performance in public libraries, concluded that of the blocked
Web pages to which library patrons sought access, between 6% and
15% contained no content that meets even the filtering products'
own definitions of sexually explicit content, let alone the legal
definitions of obscenity or child pornography, which none of the
filtering companies that were studied use as the basis for their
blocking decisions. Moreover, in light of the flaws in these
studies, discussed in detail in our findings of fact above, these
percentages significantly underestimate the amount of speech that
filters erroneously block, and at best provide a rough lower
bound on the filters' rates of overblocking. Given the
substantial amount of constitutionally protected speech blocked
by the filters studied, we conclude that use of such filters is
not narrowly tailored with respect to the government's interest
in preventing the dissemination of obscenity, child pornography,
and material harmful to minors.


To be sure, the quantitative estimates of the rates of
overblocking apply only to those four commercially available
filters analyzed by plaintiffs' and defendants' expert witnesses.
Nonetheless, given the inherent limitations in the current state
of the art of automated classification systems, and the limits of
human review in relation to the size, rate of growth, and rate of
change of the Web, there is a tradeoff between underblocking and
overblocking that is inherent in any filtering technology, as our
findings of fact have demonstrated. We credit the testimony of