Google has revealed it will let web users define the categories they are interested in for targeted advertising.

In a beta version of the new service, Google will show ads on YouTube and on its other partner sites, allowing advertisers to target surfers in broad interest categories such as sports, gardening, cars or pets, vice president of product management Susan Wojcicki said in the official Google blog.

Google currently targets advertising at surfers in one of two ways: either based on the keywords they look for using its search engine, or based on the content of web pages they visit on the sites of partners enrolled in its AdSense programme. Ads targeted in this way can only reflect surfers interests at the moment the ads are displayed, whereas the new system will allow targeted ads to be displayed on unrelated sites or in response to unrelated searches.

For example, a surfer identified as interested in running might be shown ads for new shoes even when visiting a site about cooking recipes, or when conducting a search about airfares.

"Tailored advertising does raise questions about user choice and privacy," said Wojcicki - but Google is not the first to raise such questions, she said. "Many companies already provide interest-based advertising and they address these issues in different ways."

Companies with online advertising businesses, including Google and its search rivals Microsoft and Yahoo, backed a set of behavioural advertising guidelines issued by the Internet Advertising Bureau earlier this month.

Google will allow surfers to view, delete or add the interest categories associated with their browser, and will identify the information used to serve up a greater number of the ads displayed on YouTube and on the sites of its AdSense partners, Wojcicki said. Google already identifies this information for some, but not all, of the ads it distributes.

"We will not serve interest-based ads based on sensitive interest categories. For example, we don't have health status interest categories or interest categories designed for children," said deputy general counsel Nicole Wong in a posting on Google's public policy blog.

Although the search engine is offering advertisers a new way to target potential customers, it already gathers the information about surfers' interests that will allow it to offer the service. It does so through tracking cookies placed on computers visiting its search engine or websites using its AdSense advertising network.

Ironically enough, the way that Google suggests people opt out of its cookie-based interest-tracking system is by allowing it to set a special cookie on their computers. However, the people that opt out of cookie-based tracking systems also tend to clear the cookies from their computer from time to time, which would result in Google once again tracking their interests via cookies.

To resolve this problem, Google also offers a plugin for Firefox and Internet Explorer, which will maintain the opt-out cookie even if other cookies are cleared from the browser.

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While Google will determine surfers' interests based on the sites of its AdSense partners that they visit, other companies have more ambitious plans for tracking surfers' online habits in order to sell targeted advertising.

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