The most difficult puzzle on the Internet

The conceit of advertising has always been simple: it works. Nobody has ever really known how well it works, or how to measure returns on ad spending. The Internet was supposed to change all that by enabling close measurement, but it hasn't, not really.

Technology has a way of creating as many problems at it solves. There are many problems with accurately measuring online advertising. One of them, according to the traffic-analytics firm comScore, is "viewability." To be valid, according to a white paper published by the company last week, at least half of a display ad must be visible to an Internet user for at least one second. That doesn't sound like much of a threshold, and it isn't. But at least it's something. And even by that paltry measure, nearly a third of ad impression don't measure up, according to comScore.