This question evaluates competency in A/B test interpretation, detection and validation of heterogeneous treatment effects, causal inference, segmentation analysis, and experiment diagnostics for CTR metrics.

You ran a user-level A/B test of a new ad-ranking algorithm. The reported result is a 5% overall relative lift in click-through rate (CTR), but a 100% relative lift for the subgroup "Indian males aged 18–24." Assume CTR is clicks/impressions, randomization is at the user level, and the test lasted long enough to get initial readouts.
Hint: Consider heterogeneous treatment effects, sampling bias, guardrails, segmentation, and long-term business impact.
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