Attribution
Impression
Also called ad view.
Impression is a counted opportunity to see an ad or a search result, under the rules of a measurement body. It is exposure, not attention.
How it is measured
An ad server logs an impression when the ad is requested or rendered. Search tools log one when a result appears. Standards such as the viewable impression rule add conditions about how much of the ad showed and for how long.
Ask which rule counts. Served, rendered, and viewable are three different numbers for the same placement. Use the viewable count when comparing placements.
Worked example
A regional bank buys a banner placement billed at 500,000 impressions. The ad server logs them as served. The viewability report says 54 percent met the standard of half the pixels for a second.
So 270,000 impressions were viewable. The bank negotiates a price on viewable impressions for the next flight, and the effective cost per thousand drops by about a fifth.
How it differs
An impression is an exposure. A viewability score is the share of exposures that met a standard of being seen. Impressions can be high with viewability low.
Common errors
Treating an impression as a person. Comparing counts under different rules. Ignoring frequency. Counting bot-generated impressions. Using impressions as a success measure alone.
In practice
Ask your ad vendor for both served and viewable counts. Use the viewable number in reports. Watch frequency so one person is not shown the same ad thirty times.