The document discusses factors that affect viewability rates of online display ads. It finds that: 1) Vertical ad units and larger ad sizes have higher viewability rates than smaller sizes like banners. 2) While above-the-fold placements are often assumed to be most viewable, many below-the-fold ads are actually viewed; page position alone does not determine viewability. 3) Viewability varies across content categories, with sites that hold user attention the longest having the highest rates.