When treatment leaks between units, a clean A/B lies in both directions. The interference taxonomy, when to reach for switchback vs cluster vs geo designs, and the analysis trap that makes naive standard errors useless.
Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
When treatment leaks between units, a clean A/B lies in both directions. The interference taxonomy, when to reach for switchback vs cluster vs geo designs, and the analysis trap that makes naive standard errors useless.
The screen is whether the candidate recognizes that SUTVA is broken before proposing a design, and can name the failure direction: shared inventory or pricing makes a pricing model look worse than it is, while a recommender that trains on contaminated logs can look better. The reserved follow-up is the analysis side, that switchback observations are autocorrelated so naive standard errors understate uncertainty, and you need block-level or time-clustered inference; knowing the design is half, knowing the analysis is the staff half.
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