Click fraud incidence fell to 19.1 percent in the fourth quarter of 2010. Almost one in five cost-per-click (CPC) ads are rendered useless for marketers that paid for them. CPC ads are typically short text ads that are matched to topically similar search-engine results and Web pages. Most malicious click fraud is carried out through automated botnets, but fraudsters are starting to target display ads to inflate their impression volume and thus the commissions publishers receive. In the first half of 2010, U.S. marketers spent about US$5.7 billion on CPC ads.”]
Source: https://www.csoonline.com/article/2126772/study–click-fraud-drops-slightly-in-q4.html