What's the most effective methodology for measuring the impact of paid search on other digital channels?
There are two approaches--direct and indirect. Direct would be through the use of a Multi-Touch Attribution model. With this, you can directly see what other channels were supported by the participation of PPC. Indirect is a bit trickier--you need to be able to quarantine off a control group, which wouldn't be exposed to PPC. Since most PPC isn't user-level targeted, you might have to do this with something like Geography, or segments of your product catalog. Then, you could compare the performance of the PPC-included segment versus that of the PPC-excluded. If the difference is statistically significant, then PPC helped.
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