Your question is Root-Cause Analysis for Low ROI. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
A key advertiser partner is seeing a lower return on investment for their campaigns. How would you structure your analysis to identify the root cause? Explain how you would define ROI, decompose its movement into measurable drivers, validate attribution and data quality, and distinguish campaign performance changes from changes in conversion value or business costs. Specify the data you would request, the cuts you would analyze, and the actions you would recommend based on each finding.