While the way in which filtering programs operate is conceptually straightforward – by comparing a requested URL to a previously compiled list of URLs and blocking access to the content at that URL if it appears on the list – accurately compiling and categorizing URLs to form the category lists is a more complex process that is impossible to conduct with any high degree of accuracy.  The specific methods that filtering software companies use to compile and categorize control lists are, like the lists themselves, proprietary information.  We will therefore set forth only general information on the various types of methods that all filtering companies deposed in this case use, and the sources of error that are at once inherent in those methods and unavoidable given the current architecture of the Internet and the current state of the art in automated classification systems.  We base our understanding of these methods largely on the detailed testimony and expert report of Dr. Geoffrey Nunberg, which we credit.  The plaintiffs offered, and the Court qualified, Nunberg as an expert witness on automated classification systems.  When compiling and categorizing URLs for their category lists, filtering software companies go through two distinct phases.  First, they must collect or “harvest” the relevant URLs from the vast number of sites that exist on the Web.  Second, they must sort through the URLs they have collected to determine under which of the company’s self-defined categories (if any), they should be classified.  These tasks necessarily result in a tradeoff between overblocking (i.e., the blocking of content that does not meet the category definitions established by CIPA or by the filtering software companies), and underblocking (i.e., leaving off of a control list a URL that contains content that would meet the category definitions defined by CIPA or the filtering software companies). 1.  The “Harvesting” Phase

Filtering software companies, given their limited resources, do not attempt to index or classify all of the billions of pages that exist on the Web.  Instead, the set of pages that they attempt to examine and classify is restricted to a small portion of the Web.  The companies use a variety of automated and manual methods to identify a universe of Web sites and pages to “harvest” for classification.  These methods include:  entering certain key words into search engines; following links from a variety of online directories (e.g., generalized directories like Yahoo or various specialized directories, such as those that provide links to sexually explicit content); reviewing lists of newly-registered domain names; buying or licensing lists of URLs from third parties; “mining” access logs maintained by their customers; and reviewing other submissions from customers and the public.  The goal of each of these methods is to identify as many URLs as possible that are likely to contain content that falls within the filtering companies’ category definitions.