Viewable impressions are the metric that advertisers use to quantify the percentage of ads that are actually viewed by real people. Viewable impressions is important to determine the proportion of the ad that is actually being seen.
In order to maximize advertisement revenue the viewable impressions of a website will be tracked and improved if necessary. The amount of ad displayed for more than one second is under dispute. However, it is commonly established that at least 50% must be displayed before impression is counted for the purpose of compensation. Clicking is not counted without impression because of the strong possibility of click fraud by bots or other methods of PPC fraud.