“Engagement” with a Lightbox ad on a mobile phone or tablet is achieved when someone:
watches the ad twice in a row
hovers their finger over the ad for at least 2 seconds
watches the ad for at least 2 seconds
expands or interacts with the ad
Explanations:
Lightbox ads are expandable, rich media display ads that run across the Google Display Network. engagements are charged when the user engages with the ad. For expandable ad spaces, “user engagement” means that the user hovers, clicks, or taps to expand and the ad has time to load and be seen. For pre-expanded ads (like interstitials), “user engagement” means that the user interacts with the ad after it displays.
Engagements are charged when the user engages with the ad. For expandable ad spaces, “user engagement” means that the user hovers, clicks, or taps to expand and the ad has time to load and be seen. For pre-expanded ads (like interstitials), “user engagement” means that the user interacts with the ad after it displays.
Read more here: https://support.google.com/adwords/answer/6379520?hl=en&ref_topic=3121943
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