A good way to evaluate the effectiveness of your display ads is to look at conversion statistics.
3. Other factors can also limit accuracy of view-through conversion reporting. Companies partaking in view-through conversion tracking are able to identify “post-impression” customers by locating a particular cookie on their browser. The cookie was added to the browser when the user saw the advertisement. The problem with this tracking method is that some people may delete their cookies before the completion of the tracking period (Google’s is 30 days). Also, companies won’t know you’re a view-through customer if you purchase the product or service from a computer different from the one you viewed the ad with. While both of these factors can also apply to click conversion tracking, they are still worth noting.
Photo credit: http://www.flickr.com/photos/31796655@N07/2974942783/