This ad will deliver higher click-throughs than the same ad placed at the bottom of a page. Some sites use frames to display advertising more prominently. Keep in mind that most sites have repeat visitors, and most visitors view more than a single page of information, so your 100,000 impressions could actually represent 10,000 to 40,000 unique visitors. In some cases, impressions are guaranteed, in others, estimated. Some sites will charge a flat rate for estimated impressions.
Most popular sites offer ad agency discounts and volume or frequency discounts.
A page hit, or HTML hit, is registered every time a complete HTML page, including text and graphic files is delivered. The ratio can easily be 10 or more hits or 2-4 page hits to actual impressions. Part 2 Advertising measurement is performed primarily by using tracking software and server activity logs, which meter, but understate, impressions. With caching and site mirroring, the page displaying the ad is sometimes delivered once, then stored locally on the visitor's computer or the ISP's computer. It's faster for surfers, but prevents the tracking software from counting it as an impression delivered from the Web server.