On-Line Tracking
Who Is Watching You?

Ever make a Google search and then see ads about the same subject, even on seemingly unrelated sites? Welcome to web tracking: sites continuously compiling a dossier of your on-line activities. Some people don’t mind; others find it creepy.

Firefox's Private Browsing
Cookie Monster’s Dilemma.1

The trackers’ toolkit includes cookies, invisible graphics (web bugs), flash cookies and device fingerprinting.

Cookies are small data files placed on your computer when you visit a site, either by the site itself or others such as advertisers (third-party cookies). Some are deleted when your session ends; others remain and can be updated thus tracking activities from day to day and site to site.

Web Bugs. Tiny graphics (usually 1 pixel). You can’t see them, but when downloaded they confirm that you visited a site or opened an e-mail.

Flash Cookies. Similar to regular cookies, but work with Adobe Flash. Trackers sometimes favor Flash cookies because they are not removed when you delete regular cookies and can even be used to revive deleted cookies.

Device Fingerprinting. No two systems are alike. Databases can compile system characteristics and be fairly certain when the same machine returns. Such profiles are useful for computer security as well as marketing. Panopticlick will tell you how unique your browser is.

Is This Information Anonymous? Alone it usually is, but marketers combine it with data obtained from sources such as store loyalty-cards (FaceBook),2 voter registration records (2010 Election)3 and on-line inquiries4 to target their messages.

What Can I Do? All web browsers provide tools for limiting or blocking many tracking devices. The simplest is to use your browser’s Private Browsing Mode (“Incognito” in Chrome; “InPrivate” in IE). This does not make you anonymous on-line, but does delete all regular and Flash cookies together with cached content such as web bugs every time you close your browser. The sites you visited will also not appear in your browser’s history.

Firefox's Private Browsing
Firefox’s Private Browsing.

The downside is you won’t have any browsing history to help you go back to sites you like, site settings and passwords will need to be reentered and pages might take longer to load because cached content will need to be downloaded again.

The choice is yours: most people don’t know Private Browsing exists, some use it only for certain activities, others set their browser to always open in Private Browsing Mode.

Browsers also can be set to include a “Do Not Track” request every time you visit a web site. Advertiser compliance is voluntary. A March 2013 study showed that 18% of users have this option turned on.5

An excellent source for more information is The Wall Street Journal’s series called “What They Know.” The ComputerImages web site also has pages showing the location of various browser privacy settings and links to the publishers’ support pages.

Watch What You Say. FaceBook, Twitter and the rest make ever-changing privacy promises. But as our mothers taught us years ago: The best way to keep a secret is not to tell anyone.


  1. Cartoon by Miguel Fernandez,
  2. Facebook to Partner With Acxiom, Epsilon to Match Store Purchases With User Profiles, AdAge Digital, February 22, 2013.
  3. N.H. woman targeted by ads after tracking correctly identified her as conservative interested in Republican politics and Bible. A Web Pioneer Profiles Users by Name, Wall Street Journal, October 24, 2010.
  4. On-line request let car dealer match customer’s real identity with browsing history, They Know What You're Shopping For, Wall Street Journal, December 7, 2012.
  5. “Do Not Track” Rules Come a Step Closer to an Agreement, N.Y. Times, July 15, 2013.

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Updated April 5, 2014.