One visitor can view many
ads. A raw impression is counted each time the ad is displayed to a visitor. A unique impression is counted only when a visitor views an ad within a 24 hour-period. After 24 hours, viewing this ad from this visitor is counted again as a new unique impression. The reports for unique impressions/IPs show the activities of your visitors. Counting unique impressions depends on the information from the cookies stored on visitors' browser. Without cookies, a raw impression is always counted as an unique impression.
Example
Visitor A viewed Ad1, Ad2, Ad3, Ad2. Visitor B viewed Ad1, Ad4. As a result, Ad1 has 2 unique and 2 raw impressions, Ad2 has 1 unique and 2 raw impressions, Ad3 and Ad4 each has 1 unique and 1 raw impression.
Relationship
Unique impression is always smaller than the raw number as one visitor usually sees more than one ad.
Similar terms
imp, exposure, view. Raw impression will be the default term and simply referred to as impression.
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