An ad impression occurs whenever an ad serving platform (such as DFP or OpenX) loads a display ad on a webpage. However, many impressions that are “served” are never seen by a visitor if they appear on a section of the site that requires users to scroll down from the initial load.
A viewable impression refers to an ad that is displayed in a viewer’s screen for a specified amount of time. That doesn’t necessarily mean that it is viewed–some banner blind visitors completely tune out ads–but that it was at least viewable.
Ads that are above-the-fold are visible as soon as a webpage loads, and are therefore viewable impressions (assuming that the visitor doesn’t scroll past them before the minimum viewable time is met).
Ads that appear below-the-fold will typically load along with the rest of the webpage. If, however, the visitor doesn’t scroll down to the portion of the page where these ads are located, these ads would never be viewable.
Let’s assume that the adjacent webpage, from AllRecipes.com, is loaded but that the visitor never scrolls down. As a result, only the content above the black horizontal like is ever visible.
When this page load occurs, seven ad impressions would be recorded. However, only two viewable impressions would occur: the 728×90 leaderboard and the 300×250 medium rectangle at the top of the page.
All of the ad impressions below the fold would only be considered to be “viewable” if the visitor scrolls down and puts them into the screen for a minimum amount of time.
The concept of a viewable impression can impact various metrics as well:
The concept of a viewable impression will likely become increasingly important for sites that monetize through direct ad sales.
Many advertisers and agencies are making a push to pay only for viewable impressions during CPM-based campaigns. As this becomes the common practice, it will reduce the number of impressions for which publishers are allowed to bill. This could obviously have an adverse impact on revenue, especially for sites that sell a large amount of below-the-fold under a CPM model.
If you are currently monetizing a large number of non-viewable impressions via CPM-based ad campaigns, you may need to begin considering alternatives for these units. In a few years, it is very possible that almost all advertisers will only pay for viewable impressions. This will likely set off a number of changes in the display ad industry, including a general increase of rates.