For reference, half-page ads are the most viewable format (72%), followed by billboards, or what I would call large banners (67%) with the least viewable identified as leaderboards (just 47%) which I would call smaller banners. It seems the "banner" word has disappeared from display parlance. The main reason identified for ads proving to be unviewable was slow loading of creative, typically caused by redirects and ad servers clunking away in the background at speeds not fast enough to keep up with Web users who have navigated away.
This whole issue of a billion pounds a year being wasted makes for a great headline -- but I'm not so sure. At least, I'm not so sure if they are trying to say that advertisers get charged for a billion pounds worth of ads which are deemed unviewable. Meetrics' people certainly didn't appear keen to support the figure when I challenged it. My understanding, through talking to people involved, is that the industry has made great strides in ensuring unviewable ads are not charged for. It's not universal, of course, but generally the huge media buyers going through the well-known trading desks can expect to be recompensed.
Hence, if anything, the real cost here is in missed opportunity. Credits can be applied in retrospect to avoid an advertiser paying for what they didn't receive, but the real cost is the advertiser not getting the space in the first place. Display is an awareness driver, although those in retargeting may disagree, which means the purpose of a piece of display is to make the public aware of a new launch or an offer, or to simply remind them that a brand is still there still offering the kind of products and services they need. With click-through rates of one in a thousand, the channel is clearly not one of direct response.
So when an advertiser commits budget to a campaign, they want to know that that budget will result in the ads they have bought being placed in front of the public. Having that number as much as halved obviously slashes the reach of a campaign by half. That's pretty dramatic if you're running a display campaign as the starting point to raise awareness for a new launch that you are hoping to get people interested in enough to start filling up that all-important sales funnel. Sure, a trading desk executive can retrospectively look at the figures and apply a credit. That's all well and good, but it won't help make up for a campaign that is falling short of anticipated reach.
Make no mistake. If you're dealing with a reputable trading desk or ad network the cost of viewability is far more likely to be missed reach rather than wasted budget.