a user selects two units, they are highlighted on the map, and the ratio of their values is shown at the bottom of the display (see Figure 22.2). Such precise information is not available from paper maps, although it remains to be seen how worthwhile such an option is to users The Overview function within ExploreMap includes three options: Sequenced, and Subset. Rather than presenting the entire map at once (as on printed maps), the Sequenced option the map piece by piece: The title, geographic base, and legend title and boxes appear first (in sequence), and then each class is isplayed (from low to high). Two arguments can be made for sequencing (1) it should enhance map under standing by providing users "chunks" of information (Taylor 1987), and (2) it should emphasize the quantita tive nature of the data (i.e., that the data are ordered from low to high values). However, experimental tests of sequencing have produced mixed results. A study by Slocum et al. (1990) found that although more than 90 percent of readers favored sequencing over the tradi tional static (printed) approach, objective measures of map use tasks revealed no significant difference between sequenced and static approaches. As a result Slocum et al. argued that sequenced maps might be pre ferred simply because of their novelty. A later study by David Patton and Rex Cammack (1996) revealed that sequencing is more effective when the time to study the map limited The Classes option within Overview permits any combination of classes to be displayed; for example Figure 22.3 portrays two classes of a five-class map In an evaluation of ExploreMap, Egbert (1994, 87) found that the Classes option "was liked by all subjects