Google, with its deep reservoir of data about online behavior gathered by tracking hundreds of millions of computers, is for the first time...

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Google, with its deep reservoir of data about online behavior gathered by tracking hundreds of millions of computers, is for the first time testing ways to use some of that data to aim ads at Web surfers.

Ads that a person sees on one Google search may be influenced by a previous search a few minutes earlier. Searching for “scuba,” then something else, and then “vacations” could pull up ads for diving trips, for example.

This small, but significant, change in Google’s strategy was discovered by Gene Munster, a securities analyst at Piper Jaffray, who earlier this year started a series of tests looking at which ads were displayed in a series of queries on Google’s search engine. Google assigns every computer that visits its sites a unique number — known as a cookie — and records searches and other activities in an unimaginably large file with those cookies.

The company had previously said that it had not used any of that information to draw inferences about users for the purpose of selecting ads to show them.

How data is used for advertising has become politically sensitive. Many of the biggest companies on the Internet — Yahoo, AOL and Microsoft — have bought advertising firms because they are convinced there is a lot of money to be made by tracking users’ behavior. The Federal Trade Commission is considering regulating ad targeting.