Design incrementality test for TikTok ads

Design an incrementality test, from the advertiser's point of view, to demonstrate that TikTok ads cause a lift in conversions. Specify: (1) the randomization unit (user, cookie, geo, advertiser) and your justification; (2) the holdout construction (ghost ads/PSA, pre-bid randomization, or geo holdout) and contamination controls; (3) the primary KPI (incremental conversions and iROAS) and exact formulas; (4) sample size and duration assumptions given the baseline conversion rate and expected lift; (5) how to handle cross-channel spillovers and auction dynamics; (6) guardrails (organic traffic diversion, quality) and falsification tests (AA, placebo periods); and (7) how you would report heterogeneous lift by audience and creative while avoiding p-hacking.

Overview: This question tests a data scientist's command of causal inference, experiment design, and advertising incrementality measurement in the analytics and experimentation space, requiring both practical application and conceptual understanding.

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