An ad served doesn't necessarily equal an ad viewed, and digital advertisers and publishers are catching onto this as the industry shifts toward valuing viewability—an online advertising metric that tracks seen impressions rather than served impressions. For example, if an ad is served at the bottom of the page but the user does not scroll down to see it, the ad is not counted as a viewable impression. So what affects an ad's viewability? To find out, we conducted a study of our display advertising platforms, including Google and DoubleClick. In the infographic below, we reveal five factors that affect the extent to which ads are seen, from page position to ad dimensions and more. One surprising finding: Above the fold does not always mean viewable.