There was a freakonomics podcast recently about advertising (online and traditional). No one can actually prove it has any ROI at all. No one is willing to run the experiments necessary. In the few cases of natural experiments, where ads got turned off for some people by accident, there was no change in buying behavior. https://freakonomics.com/podcast/advertising-part-1/ https://freakonomics.com/podcast/advertising-…
Not without reason. Even without the conflict of interest that Nextgrid points out in a sibling post, there's still a significant financial barrier to attempting to measure this stuff. According to a former professor of mine who spent a large chunk of his career studying this stuff, the size of study you need to conduct in order to get any kind of statistical power at all on an ROI study is just absurd. See, for example, the treatment starting on page 15 of: http://www.davidreiley.com/papers/OnlineAdsOfflineSales.pdf