Spillover, Contamination, and Concurrent Media

Synthetic control requires the donors to represent untreated outcomes. This can fail when audience delivery crosses boundaries, national media reaches both groups, travellers move between geographies, retail or supply changes differ by region, or another campaign starts near treatment.

Concurrent activity is not automatically fatal. A national or business-as-usual channel can be part of the baseline if its effect and delivery are sufficiently common across treated and donor geographies. Differential delivery correlated with treatment is the problem. Record channel, dates, spend or delivery where available, targeting logic, and expected geographic reach.

GeoSC’s interference screen compares outcome behaviour and can return signal, no_signal, or indeterminate. It has no exposure model and cannot establish whether media caused the pattern. Use campaign logs, geographic delivery data, and domain evidence. Keep the screen advisory.

If contamination is material and cannot be represented or excluded, reject the design or change the estimand. Do not reinterpret contaminated donors as valid because pre-period fit was good.