Navigation Patterns and Usability of
`Zoomable User Interfaces with
`and without an Overview
`
`KASPER HORNBÆK
`University of Copenhagen
`and
`BENJAMIN B. BEDERSON and CATHERINE PLAISANT
`University of Maryland
`
`The literature on information visualization establishes the usability of interfaces with an overview
`of the information space, but for zoomable user interfaces, results are mixed. We compare zoomable
`user interfaces with and without an overview to understand the navigation patterns and usability
`of these interfaces. Thirty-two subjects solved navigation and browsing tasks on two maps. We
`found no difference between interfaces in subjects’ ability to solve tasks correctly. Eighty percent
`of the subjects preferred the interface with an overview, stating that it supported navigation and
`helped keep track of their position on the map. However, subjects were faster with the interface
`without an overview when using one of the two maps. We conjecture that this difference was due
`to the organization of that map in multiple levels, which rendered the overview unnecessary by
`providing richer navigation cues through semantic zooming. The combination of that map and the
`interface without an overview also improved subjects’ recall of objects on the map. Subjects who
`switched between the overview and the detail windows used more time, suggesting that integration
`of overview and detail windows adds complexity and requires additional mental and motor effort.
`Categories and Subject Descriptors: H.5.2 [Information Interfaces and Presentation]: User
`Interfaces—evaluation/methodology; interaction styles (e.g., commands, menus, forms, direct ma-
`nipulation); I.3.6 [Computer Graphics]: Methodology and Techniques—interaction techniques
`General Terms: Experimentation, Human Factors, Measurement, Performance
`Additional Key Words and Phrases: Information visualization, zoomable user interfaces (ZUIs),
`overviews, overview+detail interfaces, navigation, usability, maps, levels of detail
`
`1. INTRODUCTION
`Information visualization [Card et al. 1999] has become a successful paradigm
`for human-computer interaction. Numerous interface techniques have been
`
`This work was funded in part by DARPA’s Command Post of the Future project, contract number
`F336159711018, and ChevronTexaco.
`Authors’ addresses: K. Hornbæk, Department of Computing, University of Copenhagen, Univer-
`sitetsparken 1, DK-2100 Copenhagen Ø, Denmark; email: kash@diku.dk; B. B. Bederson and
`C. Plaisant, Department of Computer Science, Human-Computer Interaction Laboratory, Univer-
`sity of Maryland, College Park, MD 20742; email: fbederson,plaisantg@cs.umd.edu.
`Permission to make digital/hard copy of part or all of this work for personal or classroom use is
`granted without fee provided that the copies are not made or distributed for profit or commercial
`advantage, the copyright notice, the title of the publication, and its date appear, and notice is given
`that copying is by permission of ACM, Inc. To copy otherwise, to republish, to post on servers, or to
`redistribute to lists requires prior specific permission and/or a fee.
`C(cid:176) 2002 ACM 1073-0516/02/1200-0362 $5.00
`
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`proposed and an increasing number of empirical studies describe the ben-
`efits and problems of information visualization, for example, Beard and
`Walker [1990], Schaffer et al. [1996], Hornbæk and Frøkjær [1999], Chen and
`Czerwinski [2000]. Interfaces with an overview and zoomable user interfaces
`have been extensively discussed in the literature on information visualization.
`Interfaces with an overview, often called overview+detail interfaces [Plaisant
`et al. 1995], show the details of an information space together with an overview
`of the entire information space. Such interfaces can improve subjective satis-
`faction (e.g., North and Shneiderman [2000]), and efficiency (e.g., Beard and
`Walker [1990]). Zoomable user interfaces organize information in space and
`scale, and use panning and zooming as their main interaction techniques [Perlin
`and Fox 1993; Bederson et al. 1996]. Research prototypes of zoomable user in-
`terfaces include interfaces for storytelling [Druin et al. 1997], Web browsing
`[Hightower et al. 1998], and browsing of images [Combs and Bederson 1999;
`Bederson 2001]. However, few empirical studies have investigated the usability
`of zoomable user interfaces, and the results of those studies have been incon-
`clusive. In addition, the usability of overviews for zoomable user interfaces has
`not been studied.
`In this article we present an empirical analysis of zoomable user interfaces
`with and without an overview. We investigate the following:
`—how the presence or absence of an overview affects usability;
`—how an overview influences the way users navigate information spaces; and
`—how different organizations of information spaces may influence navigation
`patterns and usability.
`With this work, we aim to strengthen the empirical literature on zoomable
`user interfaces, thereby identifying challenges for researchers and advising
`designers of user interfaces.
`In Section 2, we review the literature on overviews and zoomable user in-
`terfaces. Then, we present our empirical investigation of differences in nav-
`igation patterns and usability in zoomable user interfaces with and without
`an overview. Finally, we discuss the trade-off between time and satisfaction in
`such interfaces and explain the interaction between usability and differently
`organized information spaces.
`
`2. RELATED WORK
`This section summarizes the research questions and empirical findings about
`interfaces with overviews and zoomable user interfaces. It explains the litera-
`ture behind our design decisions and the motivation for the experiment, both
`described in subsequent sections.
`
`2.1 Interfaces with Overviews
`Interfaces with overviews present multiple views of an information space where
`some views show detailed information about the information space (called detail
`windows), while other views show an overview of the information space (called
`overview windows or overviews). Examples of such interfaces include editors
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`for program code [Eick et al. 1992], interfaces for image collections [North et al.
`1995], and commercial programs such as Adobe Photoshop.1 Interfaces with
`an overview have been found to have three benefits. First, navigation is more
`efficient because users may navigate using the overview window rather than
`using the detail window [Beard and Walker 1990]. Second, the overview window
`aids users in keeping track of their current position in the information space
`[Plaisant et al. 1995]. The overview window itself might also give users task-
`relevant information, for example, by enabling users to read section titles from
`an overview of a document [Hornbæk and Frøkjær 2001]. Third, the overview
`gives users a feeling of control [Shneiderman 1998]. A drawback of interfaces
`with an overview is that the spatially indirect relation between overview and
`detail windows might strain memory and increase the time used for visual
`search [Card et al. 1999, p. 307]. In addition, such interfaces require more
`screen space than interfaces without overviews.
`Taxonomies and design guidelines for overviews [Beard and Walker 1990;
`Plaisant et al. 1995; Carr et al. 1998; Baldonado et al. 2000] contain three
`main points. First, the overview and detail windows need to be tightly coupled
`[Ahlberg and Shneiderman 1994], so that navigation or selection of information
`objects in one window is immediately reflected in the other windows. Tight cou-
`pling of overview and detail views has been found useful in several studies (e.g.,
`North and Shneiderman [2000]). Second, for any relation between overview and
`detail windows, the zoom factor is the ratio between the larger and smaller of
`the magnification of the two windows. For overview+detail interfaces, this factor
`is recommended to be below 25 [Plaisant et al. 1995] or below 30 [Shneiderman
`1998]. It is unclear, however, if the sizes of the detail and overview windows
`influence the recommended zoom factor. Third, the size of the overview window
`influences how much information can be seen at the overview and how easy it
`is to navigate on the overview. However, a large overview window might take
`screen real estate from the detail window. Plaisant et al. [1995] argued that
`the most usable sizes of the overview and detail windows are task dependent.
`A large overview window, for example, is required for a monitoring task, while
`a diagnostic task might benefit from a large detail window.
`A number of empirical studies have found that having an overview improves
`user satisfaction and efficiency over interfaces without an overview. Beard and
`Walker [1990] compared the effect of having an overview window to navigating
`with scrollbars. In a 280-word ordered tree, subjects used an overview window
`that allowed dragging a field-of-view and one that allowed both dragging and
`resizing the field-of-view. For tasks where subjects tried to locate a word in
`the tree and tasks where they repeatedly went from one side of the tree to the
`other, the overview window led to significantly faster task completion. North
`and Shneiderman [2000] compared 18 subjects’ performance with a detail-only,
`an uncoordinated overview+detail, and a coordinated overview+detail interface
`for browsing textual population data. Compared to the detail-only interface, the
`coordinated interface was 30–80% faster and scored significantly higher on a
`satisfaction questionnaire. Hornbæk and Frøkjær [2001] compared an interface
`
`1See http://www.adobe.com/photoshop/.
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`with an overview for electronic documents to a fisheye and a detail-only inter-
`face. Essays produced with aid of the interface with an overview scored signifi-
`cantly higher than essays produced with the detail-only interface. However, for
`tasks that required subjects to answer a specific question, the interface with
`an overview was 20% slower compared to the detail-only interface. All but one
`of the 21 subjects preferred having the overview.
`
`2.2 Zoomable User Interfaces
`While zoomable user interfaces have been discussed since at least 1993
`[Perlin and Fox 1993], no definition of zoomable user interface has been gen-
`erally agreed upon. In this article, we consider the two main characteristics
`of zoomable user interfaces to be (a) that information objects are organized in
`space and scale, and (b) that users interact directly with the information space,
`mainly through panning and zooming. In zoomable user interfaces, space and
`scale are the fundamental means of organizing information [Perlin and Fox
`1993; Furnas and Bederson 1995]. The appearances of information objects are
`based on the scale at which they are shown. Most common is geometric zoom,
`where the scale linearly determines the apparent size of the object. Objects
`may also have a more complex relation between appearance and scale, as in so-
`called semantic zooming [Perlin and Fox 1993; Frank and Timpf 1994], which
`is supported in the zoomable user interface toolkit Jazz [Bederson et al. 2000].
`Semantic zooming is commonly used with maps, where the same area on the
`map might be shown with different features and amounts of detail depending
`on the scale. Constant density zooming [Woodruff et al. 1998a] introduces a
`more complex relation between scale and appearance where the number of ob-
`jects currently shown controls the appearance of objects, so that only a constant
`number of objects is visible simultaneously.
`The second main characteristic of zoomable user interfaces is that the infor-
`mation space is directly visible and manipulable through panning and zooming.
`Panning changes the area of the information space that is visible, and zooming
`changes the scale at which the information space is viewed. Usually, panning
`and zooming are controlled with the mouse or the keyboard, so that a change in
`the input device is linearly related to how much is panned or zoomed. Nonlin-
`ear panning and zooming have been proposed in three forms: (a) goal-directed
`zoom, where direct zooming to an appropriate scale is supported [Woodruff et al.
`1998b]; (b) combined zooming and panning, where extensive panning automat-
`ically leads to zooming [Igarishi and Hinckley 2000]; and (c) automatic zoom to
`objects, where a click with the mouse on a object automatically zooms to center
`on that object [Furnas and Zhang 1998; Ware 2000]. When zooming, two ways of
`changing scale are commonly used. In jump zooming, the change in scale occurs
`instantly, without a smooth transition. Jump zooming is used in Pad [Perlin and
`Fox 1993], Schaffer et al.’s [1996] experimental system, and commercial systems
`such as Adobe PhotoShop or MapQuest.2 In animated zooming the transition
`from the old to the new scale is smooth [Bederson and Hollan 1994; Pook et al.
`2000; Bederson et al. 2000]. An important issue in animated zooming is the
`
`2See http://www.mapquest.com/.
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`duration of the transition and the user’s control over the zooming speed, that
`is, the ratio between the zooming time and the zooming factor. Guo et al. [2000]
`provided preliminary evidence that a zoom speed around 8 factors/s is optimal.
`Card et al. [1991] argued that the zoom time should be approximately 1 s, al-
`though in some zoomable user interfaces, for example, Jazz, users can control
`both the zoom time and the zoom factor. Bederson and Boltman [1999] investi-
`gated whether an animated or jump zoom technique affected 20 subjects’ abil-
`ity to remember the topology of and answer questions about a nine-item family
`tree. Subjects were better at reconstructing the topology of the tree using an-
`imated zooming, but no difference in satisfaction or task completion time was
`found.
`The empirical investigations of zoomable user interfaces are few and incon-
`clusive. P´aez et al. [1996] compared a zoomable user interface based on Pad++
`[Bederson and Hollan 1994] to a hypertext interface. Both interfaces gave ac-
`cess to a 9-page scientific paper. In the zoomable user interface, the scale of
`the sections and subsections of the paper were manipulated, so that the entire
`paper fit on the initial screen. No significant difference was found between the
`two interfaces for the 36 subjects’ satisfaction, memory for the text, or task com-
`pletion time. Schaffer et al. [1996] compared 20 subjects’ performance with a
`zoomable user interface and a fisheye interface. Subjects had to locate a broken
`link in a telephone network and reroute the network around the link. Subjects
`used 58% more time for completing the task in the zoomable user interface.
`Subjects seemed to prefer the fisheye interface, although this was not clearly
`described in the paper.
`Hightower et al. [1998] presented two experiments that compared the his-
`tory mechanism in Netscape Navigator with a graphical history in a zoomable
`user interface called PadPrints. In the first experiment, 37 subjects were re-
`quired to answer questions about Web pages. No significant difference in task
`completion time was found, but subjects preferred the PadPrints interface. In
`the second experiment, subjects were required to return to already visited Web
`pages. Subjects were approximately 40% faster using the PadPrints interface
`and preferred PadPrints to Netscape Navigator. Combs and Bederson [1999]
`compared four image browsers: two commercial 3D interfaces, one commercial
`2D interface, and an image browser based on Pad++. Thirty subjects searched
`for images in an image database that they had just browsed. Subjects were sig-
`nificantly faster using the 2D and the zoomable user interfaces, especially as
`the number of images in the database went from 25 to 225. The study presented
`some evidence that recall of images is improved in the zoomable user interface,
`but found no difference in subjective satisfaction between interfaces. Ghosh
`and Shneiderman [1999] compared 14 subjects’ use of an overview+detail and
`a zoomable user interface to personal histories, LifeLines [Plaisant et al. 1996].
`The zoomable user interface was marginally slower than the overview+detail
`interface. No difference in subjective satisfaction was found.
`In general, the experimental results about zoomable user interfaces are
`mixed, reflecting differences in the interfaces that zoomable user interfaces are
`compared to, in the organization and size of the information spaces used, and in
`the implementation of zooming. In addition, the characteristics of zoomable user
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`interfaces and interfaces with an overview are increasingly blended. For exam-
`ple, zoomable user interfaces have been combined with transparent overviews
`[Pook et al. 2000]; some interfaces with overviews have been extended with an-
`imated zooming [Ghosh and Shneiderman 1999]; and some effort has been put
`into extending zoomable user interfaces with navigation mechanisms that sup-
`plement zooming and panning (see, for example, Jul and Furnas [1998]). The
`main difference between research in zoomable user interfaces and in interfaces
`with an overview is that research in zoomable user interfaces has investigated
`the usefulness of zooming as a way of navigating, while other research has
`focused on the impact of a coupled overview. As interfaces with an overview
`begin to use panning and zooming as their main navigation technique and as
`zoomable user interfaces begin to provide overviews and other navigation aids,
`the central research questions become (1) what is the difference between dif-
`ferent techniques for controlling and executing zooming, possibly taking into
`account the presence of an overview and other navigation aids; and (2) what is
`the effect of an overview (or other navigation aids), given that the interface pro-
`vides pan and zoom techniques. In the experiment presented next, we address
`the latter question.
`
`3. EXPERIMENT
`To understand the differences in navigation patterns and usability between
`zoomable user interfaces with and without an overview, we conducted a con-
`trolled experiment. In the experiment, subjects used interfaces we will call the
`overview interface and no-overview interface to solve 10 tasks on each of two
`differently organized maps.
`
`3.1 Hypotheses
`In addition to the three aims mentioned in the introduction, three hypotheses
`guided the design of the experiment:
`
`(1) Recall of objects on the map would be better in the no-overview interface.
`Zoomable user interfaces have been speculated to improve understanding of
`large information spaces, because of the integrated experience of the infor-
`mation space [Furnas and Bederson 1995]. As mentioned in Section 2, one
`experiment [Combs and Bederson 1999] found improved recall in zoomable
`user interfaces. In the interface with an overview, we expected subjects to
`occasionally use the overview window for navigation in the overview+detail
`interface, thereby losing the integrated experience of the information space.
`In addition, research has shown that users have difficulty in integrating
`multiple views [Card et al. 1999, p. 634]; lower recall with the overview
`interface may be one measurable implication of these observations.
`(2) Subjects would prefer the overview interface, because of the information
`contained on the overview window and the additional navigation features.
`This hypothesis was based on the research on nonzoomable interfaces with
`overviews, summarized in Section 2.
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`(3) The overview interface would be faster for tasks that require comparison of
`information objects and scanning large areas (the latter we called browsing
`tasks). The literature suggests that comparison and scanning tasks are
`particularly well supported by an overview because the overview can be
`used for jumping between objects to be compared and because it can help
`subjects to keep track of which parts of the information space have already
`been explored.
`
`3.2 Subjects
`Thirty-two subjects participated in the experiment, 23 males and 9 females.
`Subjects were recruited at the University of Maryland and received 15 US dol-
`lars for participating in the experiment. The age of the subjects ranged from 18
`to 38; the mean age was 23.4 years. Twenty-three subjects were computer sci-
`ence or engineering students, four had other majors, and five were research staff
`or loosely affiliated with the university. Thirty-one subjects used computers ev-
`ery day. Twenty-three of the subjects had never used zoomable user interfaces,
`while nine subjects had seen or used a zoomable user interface prior to partici-
`pating in the experiment. We required that subjects had spent less than 2 weeks
`in the states of Washington and Montana, because the experiment used maps
`of those states.
`
`3.3 Interfaces
`For the experiment, we constructed an overview and a no-overview interface,
`both based on the zoomable user interface toolkit Jazz [Bederson et al. 2000].
`When users held down the left mouse button, zooming in began after a delay
`of 400 ms. Users zoomed out by holding down the right mouse button. The
`maximum zoom factor was 20, meaning that subjects could view the map at
`scale 1 through scale 20. At scale 1, the initial unmagnified view of the map
`was shown; at scale 20 the initial view of the map was magnified 20 times. The
`zoom speed was 8 factors/s; that is, subjects could zoom from the initial view
`of the map to the maximum magnification in 2.5 s. Users panned by holding
`down the left mouse button and moving the mouse in the opposite direction of
`what they wished to see (i.e., the map followed the mouse). In the lower right
`corner of both interfaces was an icon showing the four compass points, which
`were referred to in some tasks. Next to this icon was a button labeled zoom out,
`which when pressed zoomed out to the initial view of the map. This button was
`expected to help subjects return to the initial view of the map if they were lost.
`The no-overview interface is shown in Figure 1. Subjects could only interact
`with this interface using the zoom and pan techniques described above.
`The overview interface is shown in Figure 2. In the top-right corner of the
`interface, an overview window shows the entire map at one-sixteenth the size
`of the detail window. This choice was arbitrary, lacking design guidelines on
`overview sizes (see Section 2.1). However, it was similar to the average size of
`the overviews we were familiar with. The current location of the detail window
`on the map was indicated in the overview window by a 70% transparent field-
`of-view box. The overview and detail windows were tightly coupled, so that
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`Fig. 1. No-overview interface showing the Washington map. The user may zoom and pan to change
`the area of the map shown. In the lower right corner of the window a button is shown that will
`zoom out to the initial view of the map. Next to this button is an indication of the four compass
`points. The colors of the map are reproduced here as different shades of gray. The map is shown at
`scale 1, that is, at the initial view of the map.
`
`zooming or panning in the detail window immediately updated the overview
`window and dragging the field-of-view box changed which part of the map was
`shown in the detail window. The subjects could also click in the overview window
`outside of the field-of-view box, which centered the field-of-view box on the point
`clicked on. The field-of-view box could be resized by dragging the resize handle
`in the bottom right corner of the field-of-view box. The subjects could also draw
`a new field-of-view box by holding down the left button and moving the mouse
`until the desired rectangle was drawn. The field-of-view box always kept the
`same aspect ratio, which corresponded to the detail window and the overview
`window.
`
`3.4 Maps
`The motivation for using maps for the experiment was threefold. First, inter-
`faces for maps constitute an important area of research. Second, maps include
`characteristics of other, commonly used information structures, for example, hi-
`erarchical information (nesting of information objects) and network information
`(connections between information objects). Therefore, results concerning maps
`may be generalized to other information structures. Third, the direct relation
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`Fig. 2. The overview interface showing the Montana map. In the top right corner of the interface
`is the overview window, which shows an overview of the entire map. The gray area in the overview
`window is the field-of-view box that indicates which part of the map is currently shown in the detail
`window. In the bottom right corner of the field-of-view box is the resize handle that allows the user
`to make the field-of-view smaller or larger, that is, to zoom in or out. The two buttons in the lower
`right corner are similar to the buttons in the zoomable user interface. The map is shown at scale 4,
`meaning that the objects in the detail window are magnified 4 times.
`
`between representation and physical reality aids interpretation of maps com-
`pared to the often difficult interpretation of abstract information spaces [Horn-
`bæk and Frøkjær 1999].
`We created two maps based on data from the 1995 United States Census.3
`The maps contained eight types of map objects: counties, cities, parks, airports,
`lakes, railroads, military installations, and other landmarks. Each map object,
`except railroads, consisted of a shape and a label. A distinct color identified each
`type of map object. In addition, county names were shown in bold type and city
`names in italic type. The maps were organized by placing labels for map objects
`at different scales, changing the apparent size of the labels as follows (also see
`Figure 3):
`—The map of the state of Washington showed map objects at three levels
`of scale: county level (scale 1, 39 labels), city level (scale 5, 261 labels),
`and landmark level (scale 10, 533 labels). At the county level, labels were the
`same size as a 10-point font when the map was zoomed out (i.e., at scale 1)
`
`3See http://www.census.gov/geo/www/tiger/ or http://www.esri.com/data/online/tiger/.
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`Fig. 3. Eight screenshots of the maps. The four screenshots in the left column show the Washington
`map; the right column shows the Montana map. From top to bottom the maps are shown at scales
`1, 3, 7, and 20. On the Washington map, map objects are labeled at three different levels: county
`level (39 counties, for example, Snohomish in the left column, screenshot 2 from the top), city level
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`and larger when the map was magnified. When labels were shown at the
`city or landmark level, they had the size of a 10-point font when the user
`magnified the map 5 or 10 times, respectively.
`—The map of Montana displayed all 806 labels at the scale 7, that is, similar
`in size to a 10-point font when the map was magnified 7 times. To aid visual
`search, county names were also shown in capital letters.
`
`The Washington map was representative of information spaces that present the
`user with rich navigation cues everywhere in the information space (such as
`Yahoo style hierarchies or well-designed semantic zooming). The Montana map
`was intended to be representative of information spaces organized in a single
`level, with weak navigation cues at low zoom factors. We originally intended
`to formally compare single-level versus multilevel maps but only created two
`maps instead of the four maps necessary to properly separate the “number of
`levels” variable (single vs. multiple) from content “noise” variable (Washington
`vs. Montana). In Section 4, Results, we therefore only report differences at-
`tributed to the map used; in Section 5.2, we speculate on the origin of the dif-
`ference of performance between maps, especially the role of map organization.
`
`3.5 Tasks
`Tasks were created to cover a large number of the types of tasks previously
`discussed in the literature [Plaisant et al. 1995] and to investigate specific
`hypotheses about when an overview would be especially useful (hypothesis 3,
`Section 3.1). We created 10 tasks for each map, five navigation tasks and five
`browsing tasks, which are described in the Appendix.
`
`—Navigation tasks required subjects to find a well-described map object. All
`of the navigation tasks specified the names of the objects to be located. In
`addition, the counties the objects were to be found in were named, greatly
`limiting the area to be searched. Two navigation tasks required subjects to
`locate an object on the map, two tasks required subjects to find and compare
`objects, and one task required subjects to follow a route between two places
`specified in the task.
`—Browsing tasks required subjects to scan a larger area, possibly the entire
`map, for objects fulfilling certain criteria. Two browsing tasks required a scan
`of the entire map for objects of a certain type; two tasks required subjects to
`scan an area of the map to find the county with the most cities or the largest
`cities in the area; and one task required subjects to find the first object of a
`certain type east of some county.
`
`Between the maps, the tasks differed only in the map objects referred to. The
`answers to the tasks were evenly distributed over the map, and answers were
`also located at different scales.
`
`(533
`for example, Everett in the lower left screenshot), and landmark level
`(261 cities,
`landmarks, barely readable in the lower left screenshot). On the Montana map, all maps objects
`are labeled at the same scale, that is, all labels are same size but can appear very small at low
`scales. At scale 7 on this map, labels are as big as a 10-point font.
`
`ACM Transactions on Computer-Human Interaction, Vol. 9, No. 4, December 2002.
`
`Apple Inc.
`Exhibit 1019
`Page 011
`
`

`

`Navigation Patterns and Usability of Zoomable User Interfaces
`
`†
`
`373
`
`We also gave the subjects two recall tasks that test their memory of the
`structure and content of the map. The first recall task consisted of five small
`maps showing the outline of the state depicted on the map. For three of these
`small maps, a part of the map was darkened and the subjects were asked to
`write down as many objects within the dark area as they remembered. For
`two of the maps, subjects themselves could mark a county on the map with a
`cross, and write down any map objects they remembered within that county.
`The second recall task consisted of three county names, each associated with a
`list of 10 cities. Subjects were told to circle all cities within a county and cross
`out cities they were confident were not located in the county mentioned. The
`list of cities consisted of the three largest cities within the county mentioned,
`the three largest cities in counties just next to the county mentioned, and four
`cities in entirely different areas of the map.
`
`3.6 Experimental Design and Dependent Variables
`The experiment varied interface type (no-overview vs. overview), task type
`(navigation vs. browsing tasks), and map (Washington vs. Montana map), in
`a within-subjects balanced factorial design. The experiment consisted of two
`parts. In the first part, subjects used one interface giving access to one map
`and performed five navigation and five browsing tasks. In the second part, sub-
`jects used the other interface in combination with the not-yet explored map.
`Subjects were randomly assig