How Do They Track You? Let Us Count the Ways

In my article in Monday’s Times, “To Aim Ads, Web Is Keeping Closer Eye on What You Click,” I worked with comScore to develop a new measure for Web companies: how much data they can collect from users.

On the Internet, companies are typically ranked by how many different people visit their sites in a given month. And when Microsoft announced its $41 billion bid for Yahoo, comScore and Nielsen Online promptly put out estimates counting how many people would be in the merged company’s total audience.

But audience size is not everything in the online world. Advertisers increasingly want media companies to find their most likely customers and show their ads only to those people, rather than to the site’s entire audience.