An online system selects different geographic regions to evaluate content presented by the online system. The online system pairs a geographic region with another geographic region based on similarity of the geographic regions. To assist this pairing, the online system identifies users associated with locations within a threshold distance of different geographic regions and retrieves demographic and other characteristics of users associated with locations within the threshold distance of one or more geographic regions. Based on the retrieved characteristics and characteristics of the geographic regions, the online system generates vectors for different geographic regions. For each possible pairing of geographic regions, the online system computes a distance value and selects a pairing of geographic regions based on the distance values. For example, the online system selects a pair of geographic regions associated with a minimum distance value.