ACoS

ACoS is ad spend divided by ad-attributed revenue. Useful for campaign-level decisions; misleading as a business metric.

ACoS tells you the ratio of dollars in to attributed dollars out, within the narrow slice of the catalogue a campaign touched. It is the right metric for comparing campaign efficiency inside the same SKU bucket.

It is the wrong metric for anything else. ACoS does not capture cannibalisation, brand-term spending you might be paying for already, or whether the customers who bought would have bought anyway. Use it for tactics, not for strategy.