Filters that use a preselected list of sites
These tools identify inappropriate Web sites by locating them on a list of sites programmed into them by the tool maker. These lists contain hundreds, sometimes thousands, of sites known to contain inappropriate content.
Depending on how the list was compiled or is updated, it might not be as precise as some families desire. Some lists are put together by "web crawler" computer programs that automatically sift through the Web, flagging sites that seem to meet their criteria for inappropriateness. These lists will often be less precise than lists compiled and maintained by human experts (see Filters using human-maintained lists). A tricky thing about preselected lists is that they can also miss sites with deceptive URL addresses.
Ultimate authority for these types of tools varies, but it may lie with the software vendor rather than with families. The lists of inappropriate sites used by some tool makers have been made publicly available, but because of their size and complexity, it can be difficult for parents to make large-scale changes in the types of content being filtered. In addition, some tool makers are reluctant to publish their lists of URL's judged to be age-inappropriate, since they don't want to provide free publicity to sites with content they consider inappropriate.
Because these tools rely on pre-existing lists of inappropriate content, their scalability is low. With all the thousands of Web pages created every day, it can take time for new pages to find their ways onto the software vendors' lists -- during which time it's impossible to restrict children's access to such sites. Some companies have approached this problem by hiring teams of experts to review sites and update lists -- see the information on human-maintained lists below.
Physical safety protections
Some of these tools offer a moderate level of physical safety protection, since they can prevent your children from venturing into dangerous areas online. The degree of physical safety protection will vary from tool to tool, depending on whether unmonitored web-based chat areas, for example, are filtered.
Filters using a predetermined list of web site addresses have variable functionaltiy costs depending on the size and breadth of the sites filtered and on whether the parent can override or edit the way the program works. Also, parents should consider whether a particular tool's breadth reflects their values; you may want a less restrictive, narrower tool to reduce the functionality costs to your children's overall online experience.