I was a bit surprised to read an article at Forbes with the title Google Faces Potential Pressure From Impression Fraud. Impression fraud is different from click fraud, in that Google is measuring ad views and not ad clicks. Impression fraud is an issue at Google because Google uses a "Quality Score" to determine the ranking of the ad.
The quality score is made of your cost per click and your click through rate (plus some other criteria that is not 100% clear to me). The click through rate is based on the number of clicks you receive for an ad and the number of views. So the more ad views, impressions, your ad receives and the less clicks you get on that ad, the lower you quality score is, the lower your ad ranks in the AdWords system.
Bear Stearns, a worldwide investment banking and securities trading and brokerage firm, kept Google's stock at an "outperform" rating due to impression fraud and the impact it can have on lower the price of keywords. In reality, the keyword prices are not directly affected by impression fraud, it is however less expensive for some advertisers to our rank their competitors. How interesting is it that Wall Street knows about impression fraud?