The most difficult puzzle on the Internet

August 20, 2012, 10:48 AM UTC

FORTUNE — 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.

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For example, an ad might appear on a page, but on the bottom half, to which a given user does not scroll. Also, nearly three-quarters of the campaigns of the major brands surveyed by comScore had at least some ads placed adjacent to “inappropriate content.” That means stuff that brand advertisers don’t what the be associated with, like porn or hate speech. (Or piracy: BMW reportedly has announced it will review its online-ad policies after anti-piracy advocates slammed it for running ads on sites offering unauthorized music downloads.)

According to the survey, between 4% and 11% of ads were served to “non-human” traffic — meaning bots. And at least 4% and up to 15% of ads were served outside their intended geographical areas.

All of these considerations should go into ad pricing, comScore says. While the unlimited inventory offered by the Internet certainly puts downward pressure on prices (as any media executive will tell you while ordering another stiff drink), some online ad placements are much better than others. “You have the economics of infinite supply at play, but it’s not really infinite,” comScore Executive Vice President Linda Abraham told ClickZ, a marketing news site. “There’s only so many great news sites out there and only so many hours in the day. Measuring “viewability” and placement “allows buyers and sellers to differentiate that inventory that matters.”

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The survey on which the white paper was based examined the campaigns of 12 major brands, including Ford (F), Kellogg (K), and Sprint-Nextel (S).