A marketer for a retailer wants to link online ad spend to physical results. What specific measurement should they implement to build data strength?

They should implement store sales measurement. Most retail revenue still happens in the shop, so a retailer judging campaigns only on website conversions is measuring a fraction of the return and will systematically underinvest in campaigns that drive footfall. Store sales measurement connects transactions made in physical locations back to the ads that influenced them, typically by importing transaction data or through loyalty and card-linked matching, so that in-store revenue appears alongside online conversions. That belongs squarely in the data strength pillar, because it changes what Smart Bidding can see: once in-store value is part of the conversion data, bidding can pursue the searches that generate shop visits and purchases rather than only those that end in an online checkout. Store visit conversions are the related, lighter-weight measure when transaction-level data is not available.