Traditional online A/B testing often breaks down in retail, marketing, and pharma because interventions are deployed at the store, region, market, or territory level rather than at the individual level. A retailer cannot show two shelf prices to two shoppers in the same aisle; a marketing team buys media at the DMA (Designated Market Area) level, where exposure is only partly controllable; a pharma team assigns field-force coverage by physician territory, not by patient.