Start at the search-term level.
Broad and Search Match traffic can hide the actual query. Sort search terms by spend, then flag those above a meaningful threshold with no install, trial, paying user, or revenue depending on the objective.
Separate measurement gaps from real waste.
A zero-revenue row may mean delayed subscription conversion, missing attribution, a newly launched campaign, or a market with different pricing. Validate the pipeline before pausing.
Use a reversible response ladder.
Add a negative keyword for an irrelevant query, reduce bids where economics are weak, or pause a clearly unproductive target. Record the reason and review performance after the change.