An impression rendered is not necessarily viewed. Real opportunity of conversion or consumer action is only on impressions that are viewable by the consumer. For example, if an ad is at the bottom of a page, it would get ‘rendered’ when the page loads but becomes ‘viewable’ only when the consumer scrolls down the page. This is called ‘Viewability’. Viewability is a metric that tells you the % of times when the ad rendered on your page is actually viewable by a visitor.
We have introduced Viewability in the new Native Shopping Ads Reports. Knowing ‘Viewability’ helps you identify opportunities to make the ads more visible and hence drive more conversions.
Viewability= Viewable Impressions/Total Impressions*100
to read more on understanding Viewability.