The old joke in the advertising business is that marketers know that half the money they spend on ads is wasted, but they just don’t know which half. Now new tools for measuring online behavior is showing which ads are remaining unviewed, but advertisers seem to be hanging on even to ads that are unlikely to be seen.

Earlier this week, the tech company AppNexus announced it will allow advertisers to pay only for online ads that people see, according to Bloomberg. Tracking technologies are making it easier to determine which ads are seen, and industry data suggests that only half of all digital ads are actually viewed by website visitors.

The move by AppNexus follows other more high-profile companies taking similar action. Facebook recently made a similar change, and Google announced that Google Display Network marketers would not be charged for ads that were not seen. The company also offered to allow independent monitoring groups to verify its claims about which ads are and are not being viewed.

Some observers think that these changes may be foretelling then end of invisible digital advertising, but AppNexus says their experience suggests that at least some advertisers will continue to buy cheaper ad space even if the ads are unlikely to be seen.

Certain ads in print publications have long been known to be virtually unseen, yet continue to draw advertisers. For example, small print ads in the back pages of major magazines are rarely if ever viewed, in contrast to prominent ads near the table of contents.

The difference with digital marketing is that companies can now identify with a high degree of accuracy which ads are not being seen. But some metrics, like measurements of page impressions, can tell when people land on a webpage but may not determine how many viewers scrolled to see ads at the bottom of a page.

Thus, marketers will continue to sell the low-page ad space, which advertisers looking for cheaper options will continue to buy. Even if the bulk of the data suggests that no one is seeing the ads.