An online system selects advertisements for inclusion in a scrollable advertisement unit that includes a display area and multiple advertisements, each associated with a position in the scrollable advertisement unit. Positions in the scrollable advertisement unit are ranked based on a measure of predicted performance of an advertisement in each position. Advertisements are ranked based on a probability of being viewed by a user if associated with a particular position in the scrollable advertisement unit and, optionally, on a probability of presenting an advertisement based on characteristics of the advertisement. The position ranking and the advertisement ranking are used to associate advertisements with positions. For example, an advertisement is associated with a position having a location in the position ranking corresponding to the advertisement's position in the advertisement ranking.