Data-driven is imperfect
Platforms overclaim. Cross-device gaps. iOS signal loss. DDA is better than last-click but still lies.
Post-purchase surveys fix the gaps
'How did you hear about us?' at checkout gives you the stated channel. Noisy but honest.
The blend
When DDA and surveys disagree, split the difference conservatively. When they agree, trust it. When they wildly diverge, find the bug.
When to invest in MMM
Above $10M annual media spend, media mix modeling pays back. Below that, the data isn't enough for MMM to produce reliable conclusions.