We only know the full truth about how viewable an ad is after the page on which it appears finishes running. Before then, the viewer might scroll to a place where an impression qualifies as in-view. Knowing in advance if an ad will be in-view is impossible – almost.

How can we bid correctly for an impression without prior knowledge that it will be in view?

AppNexus announced last week that it would provide historical placement-level viewability in bid requests, which, considering its scale, has the potential to change the digital advertising landscape.

http://adexchanger.com/data-driven-thinking/pre-bid-placement-level-viewability-will-change-the-game/