Comment by jessefrederik

Hi, I'm the author of the article, and I don't assume all online advertising is Google search. I also cite research on display advertising (meta-study of 432 display experiments on Google): https://www.ssrn.com/abstract=2701578 And on Facebook advertising: https://www.kellogg.northwestern.edu/faculty/gordon_b/files/...

The problem with all online advertising is that there are huge selection effects, which are hard to correct for using conventional statistical methods. So you need to do experiments. And when economists do experiment, they find that advertising effects are so small that they are hard to measure.