How stores use your phone’s Wi-Fi to track your shopping habits

Every smartphone these days comes equipped with a Wi-Fi card. When the card is on and looking for networks to join, it's detectable by local routers. In your home, the router connects to your device, and then voila — you have the Internet on your phone. But in a retail environment, other in-store equipment can pick up your Wi-Fi___33 card, learn your device's unique ID number and use it to keep tabs on that device over time as you move through the store.

This gives offline companies the power to get incredibly specific data about how their customers behave. You could say it's the physical version of what Web-based vendors have spent millions of dollars trying to perfect — the science of behavioral tracking. Thousands of customer interactions a day are logged and uploaded to the databases of third-party companies that specialize in retail analytics. Estimates vary as to how big this industry is, but according to Jules Polonetsky, the director of the Future of Privacy, nine major players account for the vast majority of tracking activity. Others estimate there could be as many as 40 major and minor firms.