Consider how long the ad will be shown. Normally, banner ads live on the webpage for as long as the page is live. Most publishers equate the pageview with an impression. As long as the page is on screen, the ads can be viewed. However this is not always the case with mobile apps. Some apps exhibit ads as pop-ups with only brief screen time. Those ads appear only for a few seconds then disappear from view. This is particularly true for gaming and news apps. Savvy publishers want to monetize their audiences but not overwhelmingly distract from the experience. Others simply want to run as many ad impressions as possible. Timing obviously has creative implications. Understand how long the ad will be viewable, don't assume it will appear on the page infinitely.