> Examples are bots, crawlers, more kids playing with iPads, or malicious website design, e.g, I have clicked on ads because I was tricked on it by the HTML/JS

> Examples are bots, crawlers, more kids playing with iPads, or malicious website design, e.g, I have clicked on ads because I was tricked on it by the HTML/JS Google considers all of those spam, and goes to great lengths to detect them, refund them, and remove them from the stats. Changes in spam-cleaned CTR, which is what advertisers see and what Google talks about for aggregate metrics, should not be explainable with any of those. Also, even in the short term, CTR as a primary metric only matters if you're paying a fixed amount per impression on a fixed number of impressions - i.e., almost never, unless you're looking at a particular fixed subset of the network. If prices go down, e.g. because of adding enough new inventory to decreases auction pressure, or if you add new lower-CTR inventory in a CPC auction, then the total amount of ad spend can still go up as CTR goes down. Google-wide CTR has been dropping for years, mostly due to mix shifts. In particular, Youtube ads, which have lower CTR than search, have been growing a lot faster.