It is not simple for advertisers to recognize end-users across devices. “Apple’s Safari browser for smartphones and tablets prevents tracking cookies from being installed … Many phone users jump from app to app without using their browsers … On top of everything else, “you tend to overcount clicks, because many are caused by fat fingers,” notes Kim Riedell, senior vice president of product and marketing at Digilant … Apple does use an identifier for iPhones that makes it possible for advertisers to track user activities. Google’s Android operating system permits cookies, and Google has also launchedits own Advertising ID. Even in those cases, it’s difficult to follow a user from phone to tablet to desktop unless the user logs in to an advertiser’s page on each device or uses a browser that permits tracking.”

The solution: “Recently, data management companies have started using statistical analysis to make educated guesses about user identity. They can hypothesize, based on certain limited pieces of information, that a given smartphone user probably is the same person as a given desktop user. Among the data they collect and analyze is information about the Wi-Fi networks a person uses, the websites she regularly visits, time-of-day patterns, and geographical cues.”