To determine the prevalence of Internet tracking technologies, The Wall Street Journal analyzed 50 of the most-visited U.S. websites for children and teens, as ranked by the comScore Media Metrix report from April 2010. The Journal excluded sites it had analyzed in its earlier database of major websites, and sites where fewer than 25% of visitors are under 18, according to comScore.
See more about children's privacy from the Wall Street Journal's series on Internet-tracking technology.
The Journal hired a technology consultant, David Campbell, to analyze the 50 sites for three types of tracking methods commonly used online: "HTML cookies," "Flash cookies" and "beacons." Mr. Campbell is a principal at Electric Alchemy in Denver, which specializes in software security and information assurance.
HTML cookies are small text files, installed on a user's computer by a website, that assign the user's computer a unique identity and can track the user's movements online. Flash cookies are used in conjunction with Adobe Systems' Flash software, which is widely used to display graphics and video on websites. Beacons are bits of software code on a site that can transmit data about a user's browsing behavior.
Mr. Campbell visited the 50 sites between June and August 2010. Before each session, Mr. Campbell cleared his computer of all browser data, including HTML cookies, Flash cookies and beacons. Each session consisted of visiting 20 pages per site.
Mr. Campbell used Mozilla Firefox 3.5 and Adobe Flash Player 10.0. Following each session, he examined the tracking files that had been placed on the computer.
Beacons typically don't place a file on a computer. To trace them, Mr. Campbell used Ghostery, a small piece of software that can tell if a beacon is sending information from the website being examined.
Mr. Campbell also used a network-analyzer program to record all communication during a session, and to identify when his computer connected to other sites, to download an ad, for example.
How to Protect Yourself and Your Children
Almost every major website you visit is tracking your online activity. See what you can do to protect your child's privacy . Plus, below, a step-by-step guide to fending off trackers on all sites.
Key tracking terminology
The Journal database also contains information collected by PrivacyChoice LLC about the privacy policies of companies that place these tracking files on websites. PrivacyChoice, founded by tech entrepreneur Jim Brock, provides privacy-consulting services to websites and doesn't accept money from ad companies that it surveys.
PrivacyChoice also provided the technology for the TrackerScan software that The Wall Street Journal is offering to readers to determine what cookies and other tracking tools are present on their own computers. You can access the software at WSJ.com/WTK.
The Journal compiled an "exposure index" for the 50 sites it examined, combining Mr. Campbell's findings with PrivacyChoice's analysis of cookie-placers, to determine how much each site exposes visitors to intrusive monitoring.
The exposure index gives each site a score based on eight criteria in PrivacyChoice's analysis: whether the site belongs to an industry self-regulating group; whether it lets users opt out of receiving cookies; whether it is part of an advertising or tracking network; whether it shares data it collects with others; whether it promises to keep user data anonymous; how long it retains user data; and how it handles sensitive data such as financial or health information.
A site's exposure index is the sum of the scores for each cookie, beacon and Flash cookie found on that site. The Journal used statistical analysis to group the 50 sites into four clusters of sites with generally similar characteristics.