Viewability: What you need to know
You couldn't pass through this month without falling on the hot topic of viewability. The metric of viewability tries to answer the problem of whether or not ads are viewed by applying the length of viewability against the likelihood of it actually being seen. But it is not without its drawbacks, as listed here.
These include the problem that it is too late to do anything about it after the ad has been classed as not-viewed or viewed. Also it doesn't take into account the obvious challenge of the busyness or clutter of the page. And lastly it pushes publishers to ensure viewability above other, perhaps better conditions.