Internet Filtering
by Derek L. Hansen
Internet blocking software,
commonly known as Internet filters, are designed to classify Internet
content in order to block or allow access to it. There
are a wide range of filters with varying architectures,
levels of effectiveness, and features.
Though already widely used in schools and libraries, recent federal
legislation (namely the Child Internet
Protection Act or simply CIPA)
has encouraged the use of filters by requiring their installation in
order to receive certain types of federal funding. There was some
question as to the constitutionality of CIPA, at least as applied to
libraries, but the Supreme Court upheld the law in June of 2003 (United
States v. ALA, et al. Supreme Court opinions). As many schools and
libraries scramble to comply with CIPA (or decide whether the extra
funding is worth their compliance), it has become increasingly
important to understand the
effectiveness of Internet filters and other criteria that should be
considered when selecting a specific fitlering product. I have written
a brief article titled "CIPA:
Which Filtering Software to Use?" for WebJunction (an excellent
online forum for librarians) outlining some important considerations
when selecting an Internet filter. For a more comprehensive
understanding of the effectiveness of filters and product selection see
the following discussion and resources.
Internet Filtering Effectiveness
Due the vast size of the Internet and its constantly changing nature,
it is not possible for Internet filters to perfectly classify content.
This results in two types of errors. First, under-blocking occurs when
content is not blocked that should be restricted. Second, over-blocking
occurs when content is blocked that should not have been restricted.
There is often a tradeoff between these types of errors (similar to the
tradeoff between recall and precision in information
retrieval), although specific filtering products can have both lower
under-blocking and over-blocking error rates than competing products if
their classification is more thorough, timely, and/or accurate.
The effectiveness (lack of over- and under-blocking errors) of Internet
filters has been a hotly debated issue and one of great importance as
demonstrated by the focus on empirical studies of filtering
effectiveness in the ALA case findings of fact of the District Court in
2002 (United
Stated v. ALA, et al. District Court opinion - see section 4. Attempts to Quantify Filtering
Programs’ Rates of Over- and Underblocking). Although a number
of empirical studies have been performed in the past, most of them have
suffered from methodological flaws in sample selection, classification
procedures, or implementation of blocking tests. Results have also been
interpreted inappropriately, in part because there are two independent
measures of over-blocking that are sometimes confused, and likewise for
under-blocking. For a comprehensive discussion of methodological issues
related to testing the effectiveness of Internet filters see
"Calculating Error Rates for Filtering Software" by Paul J. Resnick,
Derek Hansen, and Caroline R. Richardson, which will appear in
Communications of the ACM (for a preprint version click here).
The study "See
No Evil: How Internet Filters Affect the Search for Online Health
Information" funded by the Kaiser Family Foundation employs the
methodology recommended in the prior article. The study focuses on
health information, although a similar methodology can be used to test
the effectiveness of other types of information. Also at the Kaiser
site, there is an insightful webcast
including a presentation of the results and
a discussion of their implications. Participants in the webcast include
Caroline Richardson, M.D., University of Michigan Medical School; Vicky
Rideout, M.A., Kaiser Family Foundation; Emily Sheketoff, American
Library Association; Donna Rice Hughes, Enough is Enough; Nancy
Willard, Responsible Netizen; David Burt, N2H2 filtering company, and
Michael McGee, Planned Parenthood. Two other sources related to this
study include a research article (Richardson, et al. 2002, "Does
Pornography-Blocking Software Block Access to Health Information on the
Internet?" JAMA. 288:2887-2894) and a site including supplemental
material (e.g., source data and analysis files) related to the
study.
Selecting an Internet Filtering Product
Internet filtering software products have changed dramatically since
their inception. There are now a wide variety of products with a host
of features, making selection of a particular product difficult. For a
brief discussion of important issues to consider when selecting a
specific Internet filter see my WebJunction article "CIPA:
Which Filtering Software to Use?"
A more thorough synopsis of issues to consider can be found in my
presentation slides "Which Filter to Use?
Selecting, Configuring, and Evaluating Internet Filters" developed
for a PaLA 2003 workshop for librarians (click here to download the
entire .ppt presentation 1.5 Mg). I
have also created a brief Filtering Software Selection Aid
to help compare products and think through important issues when
deciding which product to purchase (if any). The
ALA is also in the
process of finalizing similar tools and checklists which can be found
at the ALA
CIPA site. Also,
see libraryfiltering.com for
an excellent, library-focused comparison of specific products.
Contact Information
Derek L. Hansen
University of Michigan
School of Information
email: shakmatt at umich.edu
phone: 734.764.1008
Derek Hansen's CV
Last Updated Feb. 2004