Why analyse at the ad set level instead of campaign level?
Campaigns set the budget and objective. Ad sets control the audience, placement and schedule, which are the variables that most directly affect efficiency. Two ad sets in the same campaign can have dramatically different CPMs and ROAS.
What does a high CPM but low CTR tell me?
Your ad is being shown to a broad or competitive audience but not resonating. This usually means either the audience needs tightening or the creative isn't compelling enough for that audience.
How do I identify ad set fatigue?
Look for ad sets with declining CTR over time despite consistent spend. Ad set fatigue is specific to paid social. Run this playbook weekly to catch it early.