Looking at in-browser desktop and mobile display ads served by Google Display Network and DoubleClick in July 2014, Google found vertical units were consistently the most viewable. Worldwide, 120×240 placements performed the best for viewability, with a rate of 55.6%. Units that were 240×400, 160×600 and 120×600 rounded out the top four. Meanwhile, 234×60 ads had the highest viewability rate among horizontal placement sizes; 300×250, a popular ad size, landed in last.
A variety of factors such as the types of publisher sites audited as well as the overall makeup of ad formats considered (e.g., banners vs. rich media vs. Rising Stars) are likely causes for varying viewability percentages between the two sources. Read the rest at eMarketer.