Models range from last-click (all credit to the final touch) to multi-touch and data-driven approaches. Every model is a simplification, and attribution alone tends to over-credit channels that appear late in the journey, which is why it is often paired with incrementality testing.

In practice

Pick an attribution model deliberately and know its bias. When a channel looks unusually efficient on attribution, test it for incrementality before shifting budget, because attributed credit and real lift are not the same thing.