Treated advertisers bid more, raising the control's prices
An ad platform randomly gives some advertisers a smart bidding tool and keeps others as controls, all competing in the same real-time ad auctions. Treated advertisers win more impressions at good prices; control advertisers win fewer at higher prices. The platform reports a big win for the tool.
Explain why this A/B comparison overstates the tool's value, and how you would run the experiment.
Your answer
This one is open-ended. Work it through, then check your reasoning against the full solution.