One of the great challenges of digital advertising is getting assurance that users actually see the ads for which the advertisers are paying. Traditional print media has its problems, but buying page 6 of the September issue of Vogue pretty much guarantees eyeballs. It's not the same in digital.
New metrics and standards have cropped up trying to solve this problem. In some circles the idea of the viewable impression has gained steam: measuring an ad's length of viewability as a proxy for determining the likelihood that it has been seen. Yet the metric of viewability brings troubles of its own.
Joshua Koran, formerly with AT&T AdWorks, wrote recently that the metric will not help, in part because sellers will increase prices to make up for the money lost on unviewable and thus unpaid-for impressions. Such padding would drive up costs across the board.
Three other factors work together to undermine viewability as a viable measure:
After-the-fact measurement. Viewability is measured by time spent within the creative's pixel viewports or similar methods. That means one learns whether an ad was seen only after it has run. This measurement informs other campaign impressions that will run on the same page, based on previous measurements. Using viewability, you're always at least a step behind, because when you buy an impression you don't know whether the ad will measure visibility for the particular user who's about to consume the impression; all you know is what happened on average in the past for the ad placement loaded on that page. It's a guessing game.