The published thresholds
Measurement guidance developed under the Media Rating Council with the IAB defines viewability in terms of the share of pixels on screen and the time they remain there.
| Format | Pixels on screen | Continuous duration |
|---|---|---|
| display | at least 50% of the advertisement's pixels | at least 1 second |
| large display | at least 30% of the advertisement's pixels | at least 1 second |
| video | at least 50% of the advertisement's pixels | at least 2 continuous seconds |
Two consequences follow directly. A unit that never scrolls into view can be served and still never counted. And a unit that flashes past during fast scrolling can be fully on screen without meeting the duration part of the rule.
Four ordinary reasons two systems disagree
None of these requires anyone to be wrong. They are structural, and they recur.
Different moments of counting.One system counts when the creative is dispatched; another counts when the creative reports itself rendered. Losses between those two moments belong to the gap.
Different filtering of invalid traffic.Systems apply different exclusion lists at different stages, so the same raw event survives in one total and not the other.
Different time-zone boundaries.Two systems closing the day at different hours will disagree on every single-day figure while agreeing over a long enough window.
Blocked measurement.Where the measurement call itself is prevented from loading, the delivery may still occur while the observation does not.
Published rule versus local convention
Published
- The pixel share and duration thresholds stated in the table, as defined in industry viewability guidance.
- That video carries a longer duration requirement than display.
- That the requirement is on continuous time, not on cumulative time.
A convention any team must set for itself
- Which system's total is treated as authoritative when two disagree.
- How large a discrepancy is investigated rather than noted.
- Whether a comparison is made daily or over a longer window that absorbs boundary effects.
Sources for the thresholds above
- Media Rating Council and IAB — viewable impression measurement guidelines. Source for every pixel-share and duration value in the table.
- IAB Tech Lab — OpenRTB specification. Source for the dispatch-side events referred to in the first disagreement reason.