An eyetracking study published on mediapost.com says that an ad placed above the fold* is visible to 100% of site visitors though only about 60% of them actually see it. And in case of ads placed below the fold, only 25% of your site visitors will ever see the ads.
That means 75% traffic on your site traffic will never see ads below the fold though advertisers may still have to pay for the impressions if it’s a CPM based media buy.
The ratio decreases further (diagram below) as you move ad units from the center to the sidebars or to area that are on the far left side of the page. More on
In this chart from MarketingSherpa [PDF], the bottom of the blue area represents the percent of total viewers who see an ad while the top represents the percent of total viewers who scroll to a point where the ad is visible on their screen.
This may be an hint that other than traffic and page views, the placement of ads on web pages is an equally important metric if publishers want to command higher CPM rates.
*Above the fold (definition from Economist) – The content on a Web page that is visible without scrolling. The content you see when you have to scroll down to the middle or bottom of a web page is referred to as "below the fold". This expression originally referred to the position of articles in a newspaper.