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The claim was: "67% (2/3) have negative net ROI, and so would presumably be better off with zero advertising."

If they truly have negative net ROI, they would be better off with zero spending than their actual spending. The claim holds.

> The thing to measure is the marginal ROI at zero advertising spend. Derivatives are not bounded by their average value across a range.

OK, so if we're going to get super pedantic... the derivative could be negative at the current value, and at zero, and there could be a positive ROI for some value of spending (between 0 and the amount spent, or even for some value more than what was spent). But it's not really likely, nor relevant to the point the person above made.




Furthering the pedantry, even assuming monotonically decreasing ROI as ad spending increases, as long as ROI of the first dollar was positive, there would be some optimum between zero and current levels. But my original statement still holds, zero would be preferable to current levels of spending for those brands.




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