We all assume that hidden search terms waste money, but I wanted proof. So I analyzed $20 million in spend, 14 million clicks, across 933 campaigns to see for myself. Turns out they do and to a staggering degree! For every $1 in ads you buy, Google siphons $0.85 in forced inefficiency through hidden search terms. I'm firmly in the "this is not for privacy but for Google to sell low quality inventory" camp, and the rest of the data bears that out. Beyond the lower efficiency, we also saw CPCs that were 52% higher and CTRs that were 44% lower. It was a pretty herculean task getting all this research compiled, and I've written up all my findings in a blog post, which you can find in the first comment!