The definition of a viewable impression is that at least 50% of the ad should be visible and remain visible for more than 1 second. This standard measure of ad viewability was defined by the International Advertising Bureau (IAB) to help advertisers differentiate between impressions shown to real humans and impressions shown to bots.
There are a number of potential reasons why an ad impression may be served, but the ad may not be viewable.
The ad may not be viewable because of a number of reasons, e.g.:
- The user leaves the page immediately before the ad view can be counted.
- The user sees less than 50% of the ad.
- The user uses ad-blockers.
- The page is visited by a bot.
- The user uses a browser that doesn’t serve ads.
- The user opens the page on a device that doesn’t enable ads.