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Now Playing: 01-23-12 Marketplace Tech Report

A new book explores the digital trail you leave behind when you visit a site and what advertising companies are able to determine about you based on your activities. And this kind of data mining and profiling isn't just being used to sell you detergent, it's used in political campaigns as well -- you may be seeing a different candidate than someone else sees, all based on where you went online and the trail you left behind. Also on the show, the number of tablets and readers is spiking up.