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Metrics
Impressions
The count of times an ad was served, including repeat serves to the same person. Distinct from reach, which counts unique people. Impressions divided by reach is frequency.
Why it matters
It is the denominator behind CPM and frequency, and reading it as audience size is the most common way a saturated campaign looks like a growing one.
In practice
Never read it alone. Impressions rising while reach is flat means you are buying repetition, not audience.
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