In an age where everything from friendships to financial planning has moved online, the pulse of pop culture, the ups and downs of the housing market, and even the subjects of fifth-grade homework assignments can all be measured by tallying search terms and mouse clicks.

"You can use online data to predict what consumers are doing across every facet of their lives," said Stephen DiMarco, chief marketing officer of Compete Inc., a Boston firm that tracks Web traffic. "The Web is so mainstream and so ingrained ... this is kind of like the dawn of a new age."

It also has created a new frontier for companies vying to be the Nielsen of the Net. From Nielsen//NetRatings to companies such as Compete, several companies are scrutinizing the way people surf the Net. Clients such as the US Postal Service pay for Web analyses when they want to know if receiving a catalog makes a person more likely to shop online. Telecommunications firms sign up to see whether their customers are checking out rival services.

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