What's the most effective approach to measuring the impact of display?
Most effective? Direct measurement. Clicks that lead to sales, even as part of a multi-touch path. Second most effective? Conversion correlation--you'll need a control group for this. If you're working with a display medium that allows you to specify an audience by unique id (say, email addresses into Facebook custom audiences), then you can split your list, and show ads to one group, and not the other. That's highly conclusive, too, and can account for view-through conversions without dowsing rods and tealeaves.
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