According to eMarketer , half the digital ad spending this year will be on mobile , a total of $29 billion. Advertisers want to know if their messages are reaching the right target groups of people at the right time so that ad dollars are not wasted . Some people are better targets than others for messages about, say, infant car seats, or trips to Mexico, or eye makeup, or Hummers. It is not a simple matter to measure Internet traffic, whether on the web or on mobile apps. But metrics matter to advertisers, who use them to determine the amount they are willing to pay for having their messages in a digital publication. (Versi�n de este post en espa�ol) Advertisers want to know not only the size of the audience, but its characteristics -- income, location, interests, spending habits, hobbies , and more. But for technical reasons, it is difficult to track a single user across all the devices they may use at home, at work or on the go -- smartphone, tablet, laptop, desktop. Cookies,...