Investigate why an advertiser’s spend decreased

A video ads product has two ad formats:

• Direct ads (optimize for in-platform actions)
• Brand ads (user clicks the video and lands on the advertiser’s website)

You notice that for a specific advertiser, ad spend decreased materially over the last few weeks.

Your tasks

1. Validate the drop is real (not a logging/attribution/billing artifact).
2. Quantify the change and localize it (which product surfaces, geos, devices, formats, campaigns, days/times, etc.).
3. Propose a structured approach to identify root cause(s) (e.g., demand-side budget changes vs supply/auction issues vs performance/measurement changes).
4. Explain how you would distinguish between:
   • advertiser intentionally lowering budget/bids
   • platform delivering fewer impressions (supply constraint)
   • auction competitiveness changes
   • pacing/budgeting bugs
   • tracking/attribution changes (especially for Brand ads with offsite landing)

Constraints to consider (discuss explicitly)

• Seasonality (day-of-week, holidays)
• Changes in targeting, creatives, or landing page performance
• Delayed/partial labels for offsite conversions
• Selection bias (survivorship of campaigns; paused campaigns)
• Metric tradeoffs: revenue vs spend vs impressions vs CTR vs CVR vs CPA/ROAS

Deliverable

Walk through the analysis plan, key metrics (primary + diagnostics + guardrails), and what data cuts or experiments you would run to confirm the leading hypothesis and recommend actions.