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An Introduction to Filtering

By Sarah Ormes, UKOLN, on behalf of EARL, the Library Association and UKOLN

An issue paper from the Networked Services Policy Taskgroup
Series Editor: Sarah Ormes, UKOLN

What is filtering?

Filtering is the term used to describe the use of software which restricts access to material on the Internet. Such software is often used as a method to try to prevent access to pornography and other potentially offensive material.

How does filtering software work?

Filtering software works in three main ways:

  • Keyword blocking
  • Site blocking
  • Web rating systems

Keyword blocking

Filtering packages which use keyword blocking have a list of 'forbidden words' which they search for in web pages, e-mail messages or even Internet Relay Chat and other 'chat' functions. If the software finds those words it will either prevent the user from accessing those e-mail messages or pages or block out the relevant sections of it. The list of 'forbidden words' may initially be set by the software provider although some filtering packages will allow additional words to be added by the software purchaser.

Site blocking

Filtering software packages which use site blocking have a list of sites which they will not allow access to. This list typically consists of pornography and other potentially offensive sites. The list of sites is drawn up by the software provider although usually there is an option for the software purchaser to add to it. Updates to this list of sites can usually be downloaded regularly from the software provider. Filtering packages working on this principle can also be set up to allow access only to a list of certain stipulated sites. Some of the software packages will also allow you to edit the list of 'banned' sites.

Increasingly, filtering packages use both site blocking and keyword blocking.

Web rating systems

A recent development has been the creation of web rating systems. These systems are actually built into the web pages and web browsers themselves. The most well known is called PICS (Platform for Internet Content Selection) [1] which is being developed by the World Wide Web Consortium (W3C).

The system works through web sites being rated in terms of the nudity, violence, sex and language used in them. These ratings can either be embedded in the web page itself or managed by a third party rating bureau. The web browser is then set to accept only pages which are rated at certain levels e.g. no nudity allowed but mild expletives accepted. Organisations like RSACi (Recreational Software Advisory Council on the Internet) [2] are already acting as third party rating bureaux and other organisations such as Internet Service Providers (ISPs) and filtering software companies are also developing these services.

RSACi Ratings for Language

Level 0: inoffensive slang
Level 1: mild expletives
Level 2: moderate expletives
Level 3: obscene gestures
Level 4: explicit or crude language

How well does filtering software work?

Karen Schneider's Internet Filter Assessment Project [3] evaluated how effective commercial filtering software was. Volunteer librarians gathered a collection of real reference queries and tried to answer them using workstations which did and did not have filtering software installed.

Her research found that the current generation of filters were not completely effective. They either blocked out information which was required to answer typical reference queries or did not block material which was considered by the testers to be very offensive. Filters which operated by keyword blocking alone were found to be especially ineffective:

Over 35% of the time, the filters blocked some information they needed to answer a question. Keyword blocking obscured everything from nursery rhymes ('pussycat, pussycat' - blocked repeatedly, even, in one case, when the tester used the search terms 'nursery rhymes') to government physics archives (the URL began with XXX) to the word 'button'.

Eight percent of what testers accessed they determined was 'objectionable'…filters can't block what they don't know about.

In conclusion, she found that filters were not completely reliable as yet. They did not block all offensive material and on a fairly regular basis prevented access to inoffensive and useful material. However, filters are still in a state of development and are becoming more sophisticated.

Other solutions like web rating systems are also still in an early stage of development and are too under used to be relied upon yet. Very few web sites have yet implemented PICS and general awareness of it is low but looks set to grow. Already some national governments are interested in the system and may soon be promoting it.

Why is it an issue for libraries?

Increasingly, public libraries are providing their users with access to the Internet. Some of the concerns librarians have identified about providing public access Internet services are that:

  • Users will deliberately access and view pornography

  • Children could also gain access to pornography

  • The library will, by default, be providing access to illegal material

The following section looks at some of the pros and cons of the use of filtering software in public libraries. Many of the examples drawn upon are from the experience of American public libraries. This reflects their greater experience of providing public access Internet services compared to many UK libraries.

Pros of using filtering software

Providing for users' needs

Public libraries have always had stock selection policies which guide them in selecting the best and most suitable resources for their library users. Libraries generally will not stock material that is not 'suitable' (typically, pornographic and/or potentially offensive). Providing unfiltered access to the Internet could negate the library's role in providing access to a pre-selected and organised collection.

Children and pornography

There is a possibility that children may accidentally be able to gain access to pornography and other potentially offensive and/or illegal material through the library's Internet workstations. David Burt of Filtering Facts [4] argues that:

The community entrusts its children to be safe in the library. The community assumes that the library has some minimum standards for what types of materials a child might encounter at the library. The community will not trust the library to be a safe place for children if it becomes the equivalent of an adult bookstore. Pornography and children have no business with one another, especially not in a public library.

If libraries are to maintain their perceived role as a 'safe place' for children in the community they will need to address the concerns parents may have about their children accessing pornography over the Internet. Using filtering software is one way to address these concerns and try to ensure the safety of children using public library Internet resources.

Maria's Peace of Mind

Maria's children really want to use the Internet in the nearby public library for some of their homework projects. Maria is worried about them using it as she keeps reading in the paper about the amount of pornography that is available on the Internet and how easy it is to find. When she asks the librarian in the library about this she is told that the library uses filtering software which will ensure that her children will be completely safe when they use the Internet and will only be able to see 'suitable' material. She is reassured by this and is now happy for her kids to use the Internet.

Legal issues

Libraries in the USA have had to face legal challenges when they do not use filtering software and users of the library have been exposed to 'offensive material' through library Internet connections.

In May 1998 Livermore Public Library, California became the first public library to be sued for failing to protect children from pornography [5]. The complaint was filed by a parent who claimed that a minor accessed sexually explicit web sites using the library's computers, downloaded images harmful to minors to a floppy disk, and then printed them out at a relative's house.

Cons of using filtering software

Freedom of access to information

There have been several well publicised instances of public libraries in America being prosecuted by the American Civil Liberties Union (ACLU) because of their use of filtering software [6]. The ACLU and also the American Library Association [7] argue that public libraries should provide unfiltered access to the Internet in order to maintain and uphold America's first amendment right to seek and receive all types of information, from all points of view. Libraries should provide access to the Internet with the same constitutional protections that apply to the books on library shelves.

In Britain the Library Association states in its statement on intellectual freedom and censorship [8] that:

The function of a library or information service is to provide, as far as resources allow, all publicly available information in which its users claim legitimate interest. Such provision should be regardless of format and include factual and fiction material. The materials, electronic information services, networks and other facilities provided directly or indirectly by the library or information service should be equally accessible to all users. Those who provide library or information services should not restrict this access except as required by law.

However, it should be noted that the Library Association has not yet drafted a specific policy about the use of filtering software in libraries. It is expected that such a policy will be developed in late 1998.

Who decides what is filtered?

The use of filters often involves an outside organisation developing a list of 'acceptable and unacceptable' sites. Traditionally, stock selection has been seen as a task for professionally trained staff. By accepting lists of banned sites libraries are passing over the assessment of what is and isn't suitable material for their users to an outside organisation. These assessments may not necessarily be made with the sophistication and awareness which trained librarians apply in stock selection activities.

The role of the library

The public library could be the only place where some users will be able to access the Internet. Their information needs may not necessarily map onto those used by filters and their one potential source of networked information will therefore be unusable.

Legal issues

By offering 'filtered' access to the Internet public libraries could be seen as stating that they guarantee a 'safe Internet environment' for their users. As Karen Schneider's research has shown filters are not yet 100% effective and a completely 'safe' Internet environment cannot yet be achieved. There is a danger that libraries could therefore be putting themselves into a position where they are liable for legal action when filters do not screen out inappropriate material.

David's Information Needs

David is 18 and thinks he is gay. He wants to find out about 'coming out' and talk to someone who will be able to give him advice and support. He saw a programme on television last night about coming out which gave details about a web site which looked really interesting and useful. He doesn't have access at home and so will need to use the Internet terminals in the public library. He finds a free one in the corner where no one can easily see his screen and types in the URL of the site - he gets an access denied message. He wants to know why he is getting this message but is too embarrassed to ask. He leaves the library disappointed.

Whether you filter or don't filter - have a policy!

Irrespective of whether libraries choose to use filtering software or not it is important that they develop a policy about it.

Many of these types of policies have already been developed by public libraries around the world [9]. They often vary in their detail but will generally state what users are allowed to do when using the library's Internet resources. For example, a policy may state that the Internet can only be used as a research tool i.e. not for 'chatting' or games and that parents must sign a form agreeing that they are happy for their child to use the library's Internet resources. It may also state that the library cannot be held responsible for the type of material that may be found on the Internet and that parents are responsible for supervising their child's use of the Internet.

As libraries have developed policies on most areas of their service it is important that they do so on the topic of public access Internet services. Part of that policy must be a decision about whether they implement filtering software or not. Whatever their decision it must be one that they are able to defend and explain to their users.

Example of a Library Internet Access Policy

The Bloggshire Public Library Service provides public access to the Internet in keeping with its role as a source of information, intellectual development, and enrichment for the community.

The Bloggshire Public Library Service has no control over the information accessed and cannot be held responsible for the content or quality of the information retrieved.

Transmission of any material in violation of any laws is prohibited. This includes, but is not limited to: copyrighted material, threatening or obscene material, pornographic material, or material protected by trade secret. Use for any commercial purpose is also prohibited.

As is the case with all other library materials, any restriction of a child's access to the Internet is the responsibility of the child's parent or legal guardian.

Due to our limited resources, we reserve the right to restrict Internet access to research and information sources only.


[1] PICS

[2] RSACi

[3] The Internet Filter Assessment Project

[4] Filtering Facts

[5] Livermore Public Library Court Case

[6] ACLU Court Case

[7] American Library Association Statement on Library Use of Filtering Software

[8] Library Association Statement on Intellectual Freedom and Censorship

[9] Public Library Internet Access Policies

Other relevant resources

A collection of resources about filtering can be found at:

The American Library Association's web page about the Internet and intellectual freedom can be found at:

A mailing list about filtering called FILT4LIB can be found at:
To subscribe to it you need to send an e-mail with the word "subscribe" in the subject field, and nothing in the message field.

The Loudoun software censoring case. Loudoun County, VA censorware lawsuit can be found at:


This is one of a series of issue papers which will be produced by the EARL Networked Services Policy Taskgroup. UKOLN, the Library Association and EARL member libraries participate in the taskgroup. Queries about the issue papers series should be addressed to Penny Garrod, the project manager for the initiative:

Penny Garrod
The University of Bath
Bath BA1 7AY

Telephone: 01225 826711

UKOLN is funded by the Library & Information Commission, the Joint Information Systems Committee of the Higher Education Funding Councils, as well as by project funding from the JISC's Electronic Libraries Programme and the European Union. UKOLN also receives support from the University of Bath where it is based.

EARL: The Consortium for Public Library Networking The Library Association UKOLN