In the spring and summer of 2001, the Free Expression Policy Project of the National Coalition Against Censorship surveyed all of the studies and tests that it was able to locate describing the actual operation of 19 products or software programs that are commonly used to filter out World Wide Web sites and other communications on the Internet. This report summarizes the results of that survey. Its purpose is to provide a resource for policymakers and the general public as they grapple with the difficult, often hotly contested issues raised by the now-widespread use of Internet filters. The existing studies and tests vary widely. They range from anecdotal accounts to extensive tests applying social-science methodologies. In some instances, we located only one or two test reports; in other cases -- for example, Cyber Patrol, SmartFilter, and X-Stop -- we found a great many. Most tests simply describe the actual sites that a particular product blocked when Web searches were conducted.
Nearly every one, however, revealed massive over-blocking by filtering software. This problem stems from the very nature of filtering, which must, because of the sheer number of Internet sites, rely to a large extent on mindless mechanical blocking through identification of key words and phrases. Where human judgment does come into play, filtering decisions are based on different companies' broad and varying concepts of offensiveness, "inappropriateness," or disagreement with the political viewpoint of the manufacturer.
This report was fully revised and updated in 2006. For the revised and updated report, go to http://www.fepproject.org/policyreports/filters2.pdf