If you have ever worried about specifically aimed ads that seem aware of your private moments on the Web, such as looking at sites for kitten-heel pumps, eczema medications or how to get out of debt, here is something else to fret about.
Keeping your computer free of tracking programs is not easy because of the ad industry's aggressive and sophisticated efforts, says Jeff Chester, executive director of the Center for Digital Democracy.
"It's like trying to get the room of your teenager clean," he said. "You have to do it all again the next day."
A number of tools can minimize tracking, but using them requires considerable effort and tech know-how.
Online publishers, retailers and other websites you visit often let advertising companies place cookies, a small bit of software, on your computer to track your online activity.
You can remove standard cookies using the features of any major browser, but consider deleting these ad-related cookies manually to avoid trashing those set by your favorite websites, intended to save passwords and personal preferences.
However, advertisers are increasingly using powerful software known as supercookies, such as so-called Flash and document object management (or DOM) cookies, which can hold more information, and Web bugs or beacons, which let sites record statistics like what ads attracted you to the site and whether you bought something.
They are not removed when you clear out your cookies.
To remove Flash cookies, visit Adobe's online Flash Player settings page at bit.ly/cw2roU, click on "Website Storage Settings" panel and remove all or some of the files.
Block or restrict future third-party Flash cookies by going to the "Global Storage Settings" panel.
To remove tracking programs and keep them out, it is better to enlist the help of specialized software, said Pam Dixon, executive director of World Privacy Forum.
She and other privacy advocates recommend a free plug-in known as Taco, available for both Firefox and Internet Explorer, from the privacy-software startup Abine.
Taco helps Web users manage and delete standard cookies, Flash and DOM supercookies and Web bugs. It also lets you see who is trying to follow your online movements and helps you decline targeted ads from more than 100 ad networks.
Other free browser plug-ins include:
• Better Privacy for Firefox, which removes supercookies every time you close your browser.
• Ghostery for Firefox, Chrome and Internet Explorer, which controls Web bugs.
• CCleaner, for all major browsers, which removes cookies and surfing history.
• NoScript for Firefox and the similarly named NoScripts for Chrome can block supercookies, Web bugs and ads (and some security threats).
One way to stop ad networks from gathering data is to use a new feature introduced in the Internet Explorer 8 browser called InPrivate Filtering, which stops data from traveling between you and third parties who ask for it frequently.
Note: InPrivate Filtering has to be turned on each time you fire up your browser; select InPrivate Filtering from the Safety menu.
Firefox's Private Browsing mode and Chrome's Incognito will both block cookies and stop the browser from remembering the sites you visit.
And be careful what information you give out about yourself, whether on-site registration forms, online surveys or on social networks.
Interests you volunteer will undoubtedly be used to tailor ads you see around the Web.
Web searches can also be used to inform advertisers about your likely interests.
Google says it does not use search history in directing specific ads, but both Microsoft and Yahoo do.
Slow down the marketers by spreading your searches among several engines, Dixon says.
Also consider using different companies for search and Web-based e-mail.
For instance, use Google for search if you use Yahoo Mail.
Or sign out of e-mail and clear your cookies and history before you search, so your search data and e-mail data are not connected.
Alternately, use a search engine that does not track users' activity.
Scroogle.org lets you search with Google without being tracked or seeing ads.
Startpage runs simultaneous searches on multiple engines anonymously.
Your online activity is also tracked based on the string of characters associated with your computer, known as an IP address.
If your IP address never changes, advertisers can amass a large history.
If you do not get a dynamic, or regularly changing, IP address from your Internet service provider, reset it periodically by unplugging and then plugging in your modem.
Or mask your IP address using Tor, a nonprofit service that makes online activity anonymous, or a virtual-private-network service, such as OpenVPN, which adds privacy and security by encrypting your Internet traffic, suggests Paul Stephens, director of policy and advocacy at Privacy Rights Clearinghouse.
The New York Times
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