How does anyone actually know if an ad worked?

Marketers talk about attribution, lift, and mix models like everyone already knows what they mean. This is a place to slow down and find out — starting from a plain question someone might actually ask, and working through, step by step, how you'd go about answering it. Everything here uses made-up data, so there's nothing to get wrong.

Synthetic data only — no real customer data, no uploads, no auth