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What is the definition of a viewable ad impression?

The IAB and the 3MS have established a clear definition for a viewable video or display ad impression.

Display Viewable Impression

A display impression is considered to have been viewable if at least 50% of the pixels of the creative were in the viewport for at least 1 second.

For example, in this illustration, the green pixels (oversized for demonstration purposes) are in the viewport of the browser.  The red pixels are not.  However, because more than 50% of them are in view, the ad impression is considered viewable.

For example, in this illustration, the green pixels (oversized for demonstration purposes) are in the viewport of the browser.  The red pixels are not.  However, because more than 50% of them are in view, the ad impression is considered viewable.

The shift from a served impression to a viewable impression is yet another step to greater accountability in digital media.
— IAB State of Viewability Transaction

Video Viewable Impression

A video advertisement is considered to have been viewable if at least 50% of the video player area was in view for at least 2 seconds.

Considerations

It is important to recognize that the viewability standard is centered on measuring the "opportunity to see" in much the same way that it is in print and television. Therefore, viewability does not imply anything about the incremental value of an impression other than to say that a non-viewable impression has no value.

Furthermore, a viewability is not a measure of the overall value of an impression.

Resources

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What is a CPA?