You can use geography, I assume this is meant to measure ROAS.

I've worked on a similar problem, and some very large retailers we worked with wanted to measure ROAS. They turned off all adspend in a certain large metropolitan area, while we turned on in that area. Measuring the sales lift/drop, gave an idea of the platform's effectiveness.

Any other demographic data point could also be used, not just geo. You don't necessarily need to link people directly.