Filtering systems can let too much information through
A report casting doubt on the effectiveness of filtering software has been released on the first day of a US court case challenging a federal law requiring libraries to restrict access to some net content.
The report, commissioned by the Australian Broadcasting Authority, found that many filtering programs have serious shortcomings.
It is only the latest in a series of studies that have raised questions about the abilities of such programs to shield children from all potentially harmful material on the internet.
Experts say the filtering software should never be used in isolation to control how children use the web.
Site unseen
Although filtering systems are getting more sophisticated, the vast amount of material on the web, and the speed with which it changes, is limiting their effectiveness.
Filtering systems look for keywords on webpages but now combine this with analysis of the originating servers, the domain name of a site and even the amount of skin in the images on the page.
The report found that the most effective programs were those that only allow web users access to a pre-selected list of sites.
However, it noted that they also blocked access to huge amounts of inoffensive, and potentially useful, sites.
Programs that use several tactics to classify websites, such as looking for keywords on a site and the server that page originated from, did a good job of preventing access to potentially offensive pages.
But the authors of the report noted that these programs had their failings. Many classified as safe some clearly objectionable sites.
They also excluded a lot of sites that were innocuous, and needed a lot of maintenance to keep their lists of safe sites up to date.
Some of the filtering programs gave access to more than 50% of the pornographic sites they should have blocked.
Parent power
The report said that the filtering systems fall down because they tend only to look for English words and will let through offensive foreign-language sites.
The filtering systems also missed webpages that revealed their contents via graphics rather than text.
"The software is not a substitute for good parenting practices," said Professor David Flint, chairman of the ABA.
The Commonwealth Scientific and Industrial Research Organisation (CSIRO) carried out the software testing and used over 900 websites in 28 categories to test 14 filtering programs.
Other reports have revealed problems with filtering software too.
A report prepared for the European Safer Internet project found that the performance of filtering systems could be "erratic".
Last year, a report produced for a US House of Representatives committee voiced concerns about the ability of filtering software to stop access to all objectionable material.
The report looked at peer-to-peer file-sharing systems that do not present web users with a single page. Instead, they give access to music, movies and images spread across many different computers.
Only one of the blocking programs tested in the report did a good job of blocking access to the pornography stored and shared via these peer-to-peer systems.
The American Civil Liberties Union said the report showed up the "hollow promises" of blocking-software companies that claim to shield children from every objectionable website in existence.
Poor tool
Many filtering programs take a very blunt approach to web-based systems with which currently they cannot adequately deal.
For instance, many completely block access to archive sites like the Wayback Machine which hosts databases made up of old versions of webpages.
The Censorware Project has documented the failings of net filters and their often blunt, and secretive, approach to categorising websites.
Many filtering companies rely on analysing the text on webpages for keywords or trigger phrases in close proximity to each other. Inevitably, this means that access to some pages is blocked for being offensive when they are completely innocent.
"No filter is 100% accurate," said Stephen Balkam, chief executive of the Internet Content Rating Association.
Mr Balkam said people should not rely solely on filtering programs to act as a proxy parent controlling web access.
Far better, he said, was for parents to be involved in what their children did online and to act as a final arbiter on what they could and could not see.