The Children's Partnership, 1998. Guiding Children Through Cyberspace by Carolyn Caywood. There are about 15 commercial filtering programs on the market. Filtering programs work primarily by blocking access based on a list of objectionable sites they maintain. Most vendors will not disclose their list nor the criteria they use to block sites.
Most programs allow users to add objectionable sites.
Filters often restrict access to noncontroversial sites. The American Library Association takes a strong anti-filtering position. Public libraries confront the most difficult challenges on Net access. Filtering on all public library workstations has been found unconstitutional at the federal district court level. A W3C Internet standard that rates levels of sex, nudity, violence, profanity and intolerance on a 1-5 scale.