For a number of brands and agencies, getting 100% viewability on their ad buys is not only a goal to strive toward, but a mandate with which publishers must one day comply.

This expectation is reinforced by the press, which continues to publish op-eds with headlines like, "How Will We Get to 100% Viewability? Hint: Don't Settle" and "You Wouldn't Accept 70% Performance Elsewhere, So Why Should We With Viewability?"

On the surface, this desire certainly makes sense. After all, why should a brand have to pay for ads that nobody is actually going to see? Why shouldn't publishers be able to guarantee a product that really works?

But if we dig a little deeper, it's easy to see that 100% viewability is nothing more than a fever dream propped up by brands and agencies hoping to squeeze every last cent out of the publishers they work with.

The only way such a standard would be anywhere near attainable would be if publishers across the board chose to over-deliver impressions with every campaign. And while that would certainly be nice for agencies, it's highly unlikely publishers will soon be offering this many make-goods, and frankly, it's unreasonable for ad buyers to expect them to do so.

For starters, there is just no way that any piece of technology can guarantee that an ad is seen by a real, live human. The way I think about it, viewability is 50% based on the publisher and 50% based on the will of the individual the ad is being served to. Even if an ad renders in the perfect place on the page, the human it is intended for still has the opportunity to step away from the computer to grab a snack, or go to the bathroom, or leave the house entirely. Short of breaking into someone's home and tying them to a chair until they make eye contact with a banner ad, there is simply nothing publishers can do to ensure with complete certainty that the marketing message has reached its target.

And even then, the methods we're using to determine what constitutes a "viewable impression" are deeply flawed. Right now, the industry can't even come to a consensus on what a viewable impression is. For instance, is a viewable impression one that people could view once the page has loaded, or one that is actively being viewed? What happens if the ad is viewable on the page, but someone shifts to a different tab before it can be seen? Right now, different measurement vendors are answering these questions differently. As a result, a buyer's measurement vendor and a publisher's measurement vendor can come up with significantly different estimates of what percentage of the same set of impressions were viewable. How can a publisher deliver 100% viewability when it can't even agree with buyers whether a given ad load meets this lofty standard?

Another issue is that even if these vendors were all in agreement on what was viewable, none of them is able to measure every single impression a publisher serves. In fact, according to the Wall Street Journal, most viewability vendors tell clients upfront that they are unable to measure at least 20% of all impressions. So right off the bat, publishers shooting for a total viewability guarantee would be down to selling 80% of the inventory they otherwise would have had available.

This measurement problem is especially persistent on mobile devices that come in a wide range of screen sizes. Since smartphones come in so many different shapes, it's difficult for publishers to determine in an advance whether an ad will be visible on each of them. And as the Interactive Advertising Bureau points out, the present measurement technologies are particularly ill-equipped to determine the viewability of high-impact, custom placements like home-page takeovers and roadblocks. Seeing as these ads represent some of the most lucrative real estate on any site, it would seem that any solution promising 100% viewability would need to take them into account.

For these reasons, brands and agencies should reevaluate their demands for 100% viewability and perhaps focus on what publishers have been able to give them in this department. Publishers who agree to the IAB's standard of guaranteeing 70% viewability are taking a major step forward on this front, and demonstrating a willingness to risk giving away free impressions for the sake of a stronger digital advertising ecosystem brands will feel safe investing in.

And given all these obstacles and uncertainties, what more of a viewability guarantee can brands and agencies really expect?