WSJ: “More than half the smartphone apps tested by The Wall Street Journal sent a serial-number-like identifier for the phone to tracking companies. Some tracking companies use these IDs to create profiles of cellphone users for marketing purposes. The use of these identifiers poses a greater risk than tracking technologies typically used on PC Web browsers, said Heng Xu, an assistant professor of information sciences and technology at Pennsylvania State University. This is because the numbers are difficult or impossible to delete and can be tied to other data, like a person’s location at a given moment, she said.”