MRC viewability is the standard viewability for advertisers. MRC is the Media Rating Council, a US Industry-funded organization that establishes standards for rating operations.
The requirements for Viewable Display Advertising Impressions were defined in 2014, and there are 2 requirements:
- Pixel Requirement: Greater than or equal to 50% of the pixels in the advertisement were on an in-focus browser tab on the viewable space of the browser page, and
- Time Requirement: The time the pixel requirement is met was greater than or equal to one continuous second, post ad render.
The clock starts on determining whether the ad meets the one continuous second time requirement only when the ad is determined to have met the 50% pixel threshold.
If the measurer is able to determine that there is a strong user interaction with the ad, then the ad may be counted as viewable even if it does not meet the pixel and time criteria noted above. In this context, a legitimate click (i.e., it satisfies the requirements for counting a click, based on the IAB’s Click Measurement Guidelines) would constitute a “strong user
Viewability of Large Sizes (wallpapers, billboards, skins)
For display ads sized at 242,500 pixels (equivalent to the size of a 970×250) or greater, a viewable impression may be counted if 30% of the pixels in the ad are on an in-focus browser tab on the viewable space of the browser page for a minimum of one continuous second.
According to MRC, If this 30% pixel threshold is used for display ad units of 242,500 pixels or greater instead of the standard 50% pixel threshold, this practice should be fully disclosed to data users.
Google has a viewability demo here. In the demo, users can see how the viewability and the clock works.