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`ApR l l I 3- I 8' I 9 9 6 CHI 96 \'
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`P A P E R 5
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`Using small screen space more efficiently
`
`Tomonari Kamba(l), Shawn A. Elson<2), Terry Harpold(2)
`Tim Stamper<2), Piyawadee "Noi" Sukaviriya<1)
`(1) Graphics, Visualization, and
`(2) School ofLiterature, Communication,
`Usability Center
`and Culture
`College of Computing
`Georgia Institute of Technology
`Georgia Institute of Technology
`Atlanta, GA 30332-0165
`
`tomo@cc.gatech.edu, elson@cc.gatech.edu, terry.harpold@lcc.gatech.edu,
`tstamp@mindspring.com, noi@cc.gatech.edu
`
`ABSTRACT
`This paper describes techniques for maximizing the efficient
`use of small screen space by combining delayed response
`with semi-transparency of control objects ("widgets") and
`on-screen text. Most reseru:ch on the limitations of small
`display screens has focused on methods for optimizing
`concurrent display of text and widgets at the same level of
`transparency (that is, both are equally opaque). Prior
`research which proposes that widgets may be made semi(cid:173)
`transparent is promising, but it does not, we feel, adequately
`address problems associated with user interaction with text
`that is partially obscured by the widgets. In this paper, we
`will propose that a variable delay in the response of
`overlapping widgets and text improves the effectiveness of
`the semi-transparent widget/text model. Our conclusions are
`based on usability studies of a prototype of an online
`newspaper
`that combined
`transparency and delayed(cid:173)
`response techniques.
`
`Keywords
`PDAs, icons, transparency, usability study
`
`INTRODUCTION AND PROBLEM STATEMENT
`The dramatic growth in recent years of the personal digital
`assistant (PDA) market demonstrates that users are willing to
`put up with small, hard-to-read, displays, limited storage and
`battery life, slow CPU speeds and cumbersome data transfer,
`in the hope of achieving truly portable access to electronic
`data. It is probably safe to predict that devices available only
`a few years from now will be dramatically improved. Future
`generations of PDAs will have higher-contrast, easier-to(cid:173)
`read displays, they will have greater storage capacities, they
`
`Pennission to make digital/hard copies of all or part of this material for
`personal or classroom use is granted without fee provided that the copies
`ar.: not made or distributed for profit or commercial advantage, the copy(cid:173)
`right notice, the title of the publication and its date appear, and notice is
`given that copyright is by permission of the ACM, Inc. To copy otherwise,
`to republish, to post on servers or to redistribute to lists, requires specific
`pennission and/or fee.
`CHI 96 Vancouver, BC Canada
`@ 1996 ACM 0-89791-777-4/96/04 .. $3.50
`
`will be much faster and run for longer periods of time
`between charges, and they will be more flexible in how they
`communicate with each other and with other computing
`devices. These probable changes in PDAs are, however,
`limited by two constraining factors: the dimensions of
`displayed text and of the screens on which the text is
`displayed are unlikely to change very much. Reductions in
`the size of displayed text will be limited by the ability of
`users to discem small type sizes on any display device,
`especially one of relatively low resolution. Our informal
`observations of PDAs based on the Apple Newton and
`General Magic's Magic Cap operating systems suggests that
`a practical
`threshold of legibility for most users lies
`somewhere between 9 and 12 points (72 points = 2.5 em.)
`The small screen size of PDAs is not a technical limitation,
`but a key factor in their usefulness. Users clearly want
`devices that can be easily carried about and held in one hand.
`It is not difficult to imagine even smaller PDAs, worn on the
`wrist or carried on a key chain.
`
`The small physical size of a PDA limits the maximum size
`of its screen, which can be no larger than the dimensions of
`the machine in which it is embedded. On the other hand, the
`need for displayed text to be legible defines another, more
`subtle boundary: if the size of text cannot be reduced below a
`threshold of legibility, then, as the screen shrinks in size, and
`less information may be shown on it, and the user will be
`required to increase the level of interaction with the device in
`order to get to desired information.
`
`The design of user interfaces for PDAs must balance two
`opposing forces: the need to shrink the screen to a size that
`fits inside a very small box (we'll call this the "physical"
`limitation), and the need to keep the screen sufficiently large
`to show enough information that the device is actually useful
`(we'll call this the "functional" limitation.) This balance
`becomes particularly difficult in the case of navigational or
`functional controls, the widgets that must somehow be made
`available to the user to allow interaction with the information
`on-screen (switching between tasks, selecting information to
`be changed in some way, adding new information, etc.).
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`Most of the interface objects used in desktop computing
`environments - pull-down or popup menus, multiple
`windows, icons - consume a great deal of valuable screen
`space on the PDA screen. This accounts, we believe, for the
`efforts of the designers of the current crop of PDAs to
`minimize the size of these interface objects, or, in some
`cases, to eschew them altogether. It explains also the
`emphasis that many of these systems place on handwriting
`recognition technologies - the keyboard is perhaps the most
`cumbersome of control widgets. (The document-oriented,
`pen- and gesture-driven elements of the Newton interface are
`a good example of these approaches.)
`
`Once again, however, the interface designer faces a version
`of the functional limitation: the user will expect not only to
`passively read content displayed on the device, but also to do
`something with it (to select it, modify it, or enter new
`content, etc.), and this means that he or she will need to
`interact with objects (widgets) that are different from the
`text, but equally available. These widgets must be large
`enough to be readily distinguished from the content and from
`each other, and to be practical targets for some kind of
`interaction (by means of a finger or pen, for example.) But if
`they are displayed all or most of the time, they will consume
`screen space that could otherwise be used to display content.
`Because the number of these widgets will depend on the
`functions supported by the active application, and not on the
`size of the screen, the portion of screen that must be
`surrendered to them will increase as the screen grows
`smaller, as illustrated in Figure 1.
`
`i 0mmO?J
`
`(b) small screen
`
`i
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`..................... !
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`(a) large screen
`
`FIGURE 1. Surrendering screen space to widgets
`
`The price paid for showing control widgets can be
`substantial - in the Magic Cap interface, for example, nearly
`25% of the screen is used to display icons along the bottom
`and right edge of the screen.
`
`Our research focused on techniques for reducing the screen
`to widgets,
`thereby
`that must be surrendered
`space
`maximizing the available space for display of content. In the
`next section, we will review general solutions to this
`problem suggested by the work of other researchers and
`designers working with interfaces of both handheld devices
`
`CHI 96 APRIL 13-18, 1996
`
`and desktop computers. Among these solutions, we will
`single out one approach that seems especially promising to
`us: the use of semi-transparent widgets, laid over text, so that
`both are present on the screen at the same time, but the text is
`able to fill nearly the entire screen. We will, however, point
`out a serious limitation with this solution, related to user
`interaction with the text. In the remaining sections of the
`paper, we will propose a variation of the semi-transparent
`widget/text model that improves its responsiveness to user
`interaction, and describe the results of a series of usability
`studies of a prototype that applies this improved model.
`
`RELATED WORK
`
`Showing Information Structure More Efficiently
`One approach to the problem of maximizing the display of
`content is to improve the efficiency by which its underlying
`structure
`is
`represented on-screen. Several
`tools
`in
`Information Visualizer [ 4] take this approach. Cone Tree
`[13] and Hyperbolic Geometry [8] display in lucid forms
`complicated data hierarchies
`that might be otherwise
`invisible to the user, and Perspective Wall [10] does much
`the same for complex linear data structures. Cone Tree
`represents data hierarchies in 3D cone-shaped graphics.
`Hyperbolic Geometry displays a focused point within a data
`hierarchy in a large bounded space, and its context in a
`smaller bounded space. Perspective Wall displays relations
`between different nodes within the same document on two
`adjacent planes ("walls"), with semantic or structural
`differences between the nodes represented by the relative
`positions of the nodes on these walls. These methods for
`representing underlying data structure are arguably more
`precise and efficient than, for example, the very concrete
`"desktop" paradigm of Magic Cap (in which, for example,
`the Datebook application is accessed by tapping on a
`miniature appointment book), but they do not address the
`functional limitation of simultaneous display of widgets and
`content on a small screen.
`
`Showing Information Content More Efficiently
`A second approach to the problem directly addresses the
`presentation of the displayed content. Magic Lens [3] is a
`filtering tool that can change or modify the presentation
`strategy of objects on the screen over which it is laid. For
`example, a portion of a geographical map can change into a
`weather map or a population map when a special lens is
`applied to it. Starfield [6] displays a two-dimensional
`scatterplot of a multidimensional database, applying a
`dynamic filtering mechanism which continuously controls
`the density of information shown on the screen, and panning/
`zooming techniques to focus on the portion of the content
`displayed. Table Lens [11] represents large databases in
`table format. The user can zoom in on an arbitrary part of a
`table, and view it alongside a global view of the table.
`
`Pad++ applies another kind of zooming/panning technique
`to displayed content [2]. The user can focus on and zoom
`into any part of the content, and smooth animation during the
`zoom insures that the semantic link between the zoomed
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`APRIL 13-18, 1996 CHI 96 '~
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`PAPERS
`
`information and its context are maintained. Galaxy of News
`[ 12] uses similar zooming
`techniques combined with
`dynamic restructuring of data hierarchies during
`the
`zooming/panning process.
`
`We feel that these methods suggest several improvements on
`the simple, scrolling displays of long lists or blocks of text
`that are commonly encountered when using PDAs.
`However, they suffer from a critical limitation: filtering
`information content so that it may be displayed in smaller
`chunks that require less screen space does not obviate the
`need for widgets required to apply the filters, or to enable
`manipulation of the filtered content.
`
`Showing Widgets More Efficiently
`We were surprised to find little research that directly
`addresses the efficient display of the widgets themselves.
`We suspect that this may be because the physical/functional
`opposition outlined above is both obvious and intractable -
`designers know that widgets can only be so small before they
`cease to be useful, so they have concentrated their efforts on
`increasing the intelligence or functionality of the widgets,
`rather than how much screen space must be sacrificed
`because of them.
`
`Gestural interfaces, like that used in Apple's Newton
`operating system, mark an important departure from the
`paradigm of on-screen control objects. Tying functionality to
`pen-based gestures frees up valuable screen space, because
`the user can directly interact with displayed content by
`simply drawing over it with the pen. This technique works
`well in many applications, but it also has limitations. First,
`the user must learn the gestures, and be able to reproduce
`them accurately. (The list of gestures on the inside of the
`screen covers of the Newton MessagePad 110 and 120
`suggests that users are unlikely to be able to recall all the
`possible gestures without a mnemonic aid.) Second, not all
`functions supported by current PDAs can be mapped to
`intuitive gestures. There is a row of unchanging iconic
`buttons below the active screen space of the MessagePads.
`These buttons are used to switch between applications, and
`to undo previous tasks. By moving these buttons out of the
`active screen space, Apple has reclaimed area for displaying
`information, but in so doing has also underscored the
`difficulty of the problem illustrated in Figure 1.
`
`Kurtenbach and Buxton's Marking Menus [7) offer an
`interesting variation on the gestural interface. In their
`system, when the user holds a pen to the screen for a
`predefined period of time, a pie-shaped menu is displayed
`below the pen, and the user can select an item in the menu by
`stroking toward it. Users who are familiar with the structure
`of the menu can select menu items without waiting for the
`pie to be displayed, by simply stroking in the direction of
`that slice of the pie. This approach appears to combine
`strengths of a gestural interface and a menu- or icon-driven
`interface, as it hides widgets until they are needed, and
`allows the content to fill the entire screen. For these reasons,
`Marking Menus may be a good solution for reclaiming space
`on small screens. It does not, however, permit simultaneous
`
`A
`
`(a) opaque widgets
`
`(b) semi-transparent
`widgets
`
`Figure 2. Selecting content beneath
`semi-transparent widgets
`
`display of widgets and content. Because the pie menu is
`opaque, it will at least temporarily obscure underlying
`content.
`
`Semi-transparent Widgets And Their Limitations
`An approach that begins to address this objection makes use
`of semi-transparent popup menus that do not completely
`obscure underlying content [5]. (Lieberman, et al. have used
`a similar semi-transparency method to show data context in a
`zooming interface [9].) Harrison, et al. proposed allowing
`users to define the level of menu transparency. They showed
`that the transparency levels selected by users will vary,
`depending on the tasks to be completed, and each user's
`level of expertise.
`
`We believe that the use of control widgets of variable
`transparency is a promising method for maximizing usable
`screen space. If the transparency of widgets is adjusted so
`that the content with which they intersect on-screen is
`nonetheless legible, then it should be possible to display both
`the widgets and a full screen of content without sacrificing
`any screen space reserved for the latter. There is, however, a
`serious obstacle that must be overcome before this method
`will support user interaction with the content.
`
`For the purposes of our argument, let us assume that the
`widget layer is on top of the content layer. If opaque widgets
`obscure otherwise selectable content, and no mechanism is
`provided for passing through the widget layer to the content,
`then the user will be able to interact with only those parts of
`the content which are not obscured. If, however, the widget
`layer is semi-transparent- that is, if the underlying content
`and the widgets are legible at the same time- then it is not
`immediately clear which of the two layers is selectable (for
`copying, editing, etc.), even if there is no formal mechanism
`for selecting the bottom layer. This is illustrated in Figure 2.
`
`One solution to this problem is to tum off the selectability of
`underlying objects, even when they are visible. However,
`this is likely to confuse and frustrate users, who will
`reasonably expect that visible objects will sustain some
`degree of interaction, simply because they are visible. It is,
`moreover, fairly easy to build a scenario where this solution
`will effectively prevent underlying objects from ever being
`selected - if, for example, the upper layer cannot be moved,
`and the lower layer cannot be scrolled so as to move the
`partially-obscured content into a selectable region of the
`screen. The only course of action in this case would be a
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`hardware toggle of some kind to force selectability - an
`awkward exception to the normal behavior of the interface
`and an unnecessary extra task for the user.
`
`Instead, we propose a technique for determining the layer
`receiving user interaction that does not require an additional
`modifying step, as it based upon variations in the duration of
`the interaction. In this variation on the transparent widget/
`content model, the length of time during which the user
`engages with a region of the physical screen (wherein
`widgets and content overlap) would determine which virtual
`layer of the screen receives the interaction.
`
`THE EXPERIMENT
`
`Overview
`To evaluate the merits of this technique in a practical
`example, we decided upon a prototype emulating a text(cid:173)
`based online newspaper. PDAs are widely used to read text
`downloaded from digital news sources. These applications
`involve moderate quantities of relatively unstructured text,
`and usually support limited interactivity - simple navigation
`between stories, or between issues, searching for text strings,
`transferring information from a story to another application
`(for example, a scrapbook), and hypertext linking between
`stories or issues. The emulation of hypertext linking offered
`a focused example of object selection through layers - how,
`we wondered, would users attempting to select a passage of
`"hot" text (that is, a hypertext link) that was partially
`covered by a semi-transparent icon (or vice-versa) manage
`the transition between layers?
`
`The prototype was implemented on a Macintosh in Allegiant
`SuperCard. Although the mouse-driven interface of the
`Macintosh differs from the pen-based interfaces of many
`PDAs, we believed that using a desktop computer for initial
`evaluation would nonetheless generate valuable data.
`
`Daily News
`
`Click on the newspaper to browse world,
`national and business news summaries.
`Click on the desktop icon to quit.
`
`CHI 96 ApRIL I 3- I 8. I 9 9 6
`
`Though we have emphasized in this paper the importance of
`the issue of screen size for PDAs, the balancing of window
`size with maximum content display in small windows is of
`value on desktop computers, where multiple windows may
`be open at any one time, and the user may wish to reduce the
`size of any given window to a practical minimum, while still
`being able to display as much content as possible. Interaction
`techniques developed for small screens would in this case
`apply equally well to larger screens displaying many small
`windows. Moreover, this initial run of the experiment was
`designed to test the merits of varying the responsiveness of
`semi-transparent objects (widgets and text), and those kinds
`of objects need not exist only on PDAs.
`
`The overall design of the prototype strongly resembled that
`of AT&T' s PersonaL ink news reader for Magic Cap devices
`(see Figure 3). The active screen region in the prototype was
`320 x 240 pixels. The top portion of most screens in the
`prototype contained labels identifying the current story and
`icons used to move between stories or to return to an index
`for that issue of the newspaper. The text of the story filled
`the center of the screen. Hypertext links in the story were
`underlined. Along the bottom of the screen were seven icons
`(shown left to right in Figure 3): Desk (to quit the newspaper
`and return to a hypothetical desktop), Search, Archive (to
`display previous issues of the newspaper), Append (to copy
`the current story to a Scrapbook), Scrapbook (to open a
`hypothetical Scrapbook application), and Trash. Along the
`right edge of the screen were icons for moving "up" or
`"down," to the next or previous page of the current story.
`
`Most of the icons, and all of the hypertext links were only
`partially functional - when the user successfully highlighted
`a link or icon, a status message was briefly displayed in a
`floating palette on the screen. (This palette is not shown in
`Figure 3.)
`
`....
`-<1111 Story 4 of6
`Delegates agree to limit overfishing
`
`l!llt1 Newslndex
`
`United Nations - A global treaty to prevent overfishing ond\
`the high seas won approval from U.N. delegates from 100
`•·••··••·
`nations who plan to adopt the document today,
`
`The 31-page document also would permit the boarding of
`vessels that violate fishing regulations, said Satya Nandan,
`chairman of a U.N. conference on the toeic.
`
`NMillions of people in coastal communities will benefit," said
`Brian Tobin, Canada's minister of fisheries and oceans. NThe
`world will have pulled back from the edge of destroying .. , the
`living resources of the ocean,"
`•·•••·•
`
`+~~i~v·.·.·.~r~.·.··.·.·.·;.~.•.e.··.··~~.•.:.6.1.1r.··.b.~.·.· .. · .... : .. ,G.•~;~~·~~r· ········.···,·•·•····:··········•·•·•·•··· ....... ·····r····•••·····
`,. ,_
`g ~)e'~,t~gr~ -. .. ~~~.
`
`Figure 3. Typical screens in the prototype
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`APRIL 13-18, 1996 CHI 96
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`The layered icons and text at the bottom of the screen -
`comprising nearly 20% of the total screen space -were of
`variable translucency. As the icon layer was made more
`translucent, the text layer became more opaque, and vice(cid:173)
`versa. After a small series of pilot tests, we decided to fix the
`translucency settings at 80% opacity for the text, and 20%
`for the icons, to insure good legibility of both icons and text.
`As we were principally interested in recording the test
`subjects' reactions to changes in the responsiveness of links
`and icons, we did not allow the subjects to change the
`transparency settings, nor did we run the test at other
`transparency levels.
`
`Sixteen volunteer subjects were selected from among the
`faculty, graduate students and staff of the Graphics,
`Visualization and Usability Center, and the School of
`Literature, Communication and Culture of the Georgia
`Institute of Technology. Most of the subjects were expert
`users of the Macintosh or another mouse-driven graphical
`user interface. Most were familiar with hypertext concepts.
`Fewer than half had any experience with PDAs.
`
`to
`the prototype
`After a brief training session with
`demonstrate the functions of the semi-transparent icons and
`the delayed response behavior of the software, each subject
`was asked to perform eight tasks:
`
`1) From the story index (not shown in Figure 3), select a
`story title overlapping an icon. (Selecting a story title jumps
`the user to the first page of that story.)
`
`2) From the story index, select a story title that does not
`overlap an icon, scroll two pages into the story, and select a
`link overlapping an icon.
`
`3) From the story index, select a story title that does not
`overlap an icon, and select an icon that does not overlap with
`any linked text.
`
`4) On the same page, select a link that does not overlap with
`any linked text.
`
`5) From the story index, select a story title that does not
`overlap with an icon, scroll three pages into the story, and
`select a link overlapping an icon.
`
`6) From the story index, select a story title that does not
`overlap with an icon, and select a link overlapping an icon.
`(The "Greenpeace" link shown in Figure 3.)
`
`7) From the story index, select a story title that does not
`overlap with an icon, And select an icon that does not
`overlap with any linked text.
`
`8) On the same page, select an icon that does not overlap
`with any lip.ked text.
`
`(The actual instructions given to subjects were more specific
`than those listed above.)
`
`Each subject repeated the eight tasks six times: three times
`with the prototype configured to favor link selection (that is,
`to initially highlight the link when the mouse was clicked
`where a link and an icon overlapped, as if the link were on
`top of the icon), and three times with the prototype
`configured
`to favor
`icon selection. In both selection
`methods, if the mousebutton were held down for longer than
`a predefined period of time, the click would appear to pass
`through the object on top, highlighting the object under it.
`The association between response delays and layers was
`rigorously enforced: if selection of links was favored, the
`subject was required to hold down the mousebutton for the
`stipulated period of time before any icon would be selected,
`even if the icon did not overlap with any links.
`
`We further divided the link-first and icon-first selection
`groups according to the response delay that determined the
`switching between layers. Each subject attempted all eight
`tasks for each method of selection with preset delays of 215
`second and 4/5 second. In the third set of tasks for each
`selection method, the subjects were allowed to adjust the
`response delay, between a range of l/5 of a second and a full
`second. The sequence in which the subjects performed the
`series of tasks was shuffled to compensate for learning
`effects.
`
`Task 1 Task2 Task3 Task4 TaskS TaskS Task7 TaskS
`
`Total
`
`Links-first, 2/5 sec
`
`Links-first, 4/5 sec
`
`Icons-first, 2/5 sec
`
`Icons-first, 4/5 sec
`
`Total
`
`9
`
`II
`
`7
`
`2
`
`29
`
`3
`
`2
`
`16
`
`11
`
`32
`
`5
`
`9
`
`8
`
`6
`
`28
`
`3
`
`1
`
`0
`
`0
`
`4
`
`...
`
`4
`
`7
`
`5
`
`7
`
`0
`
`1
`
`8
`
`3
`
`2
`
`3
`
`6
`
`1
`
`23
`
`12
`
`12
`
`1
`
`1
`
`0
`
`0
`
`2
`
`27
`
`35
`
`50
`
`30
`
`142
`
`TABLE 1. Raw error counts
`("Links-first 2/5 sec" = Links selected first, with a 2/5 second delay between icon and link layers)
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`i
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`!links-first, 2/5 sec
`
`Links-first, 4/5 sec
`
`Icons-first, 2/5 sec
`r--
`Icons-first, 4/5 sec
`
`0.731
`
`0.361
`
`1.323
`
`0.424
`0.833
`1.469 I 0.794
`1.354
`0.800
`
`0.250
`
`0.000
`
`0.000
`
`1.053
`
`0.388
`
`0.793
`
`0.355
`
`0.513
`
`0.000
`
`Task 1 Task 2 Task 3 I Task4 Task 5 !Task 6 1 Task 7 TaskS
`0.14~~-077
`0.234
`0.125
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`CHI 96 ApRIL I 3- I 8.
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`I 9 9 6
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`0.641
`0.077
`0.504 I 1.089
`0.664
`0.302
`
`0.609
`
`0.000
`
`0.167
`
`0.000
`
`Total /
`
`3.327
`
`3.907
`5.196 I
`3.648
`
`Total
`'
`
`3.468
`
`3.635
`
`3.220
`
`0.605
`
`2.322
`
`1.468
`
`1.158
`
`0.202
`
`16.078
`
`Table 2. Normalized error counts
`
`("Links-first, 2/5 sec" = Links selected first, with a 215 second delay between icon and link layers)
`
`At the conclusion of the experiment, each subject was given
`a brief questionnaire that reviewed the test, and included
`questions asking which of the selection methods was
`preferred (links first or icons first), and which switching
`delay (between 1/5 and I second) was preferred for each
`method.
`
`The error counts shown in Table 2 have been normalized so
`that each subject's contribution to the total errors
`is
`(Standard deviations were calculated for each subject to
`insure that there were no outliers included in the data.)
`Figure 4 shows a simple line graph of the data in Table 2.
`
`The Results
`As the subjects attempted the eight tasks, the prototype
`recorded every mouseclick,
`tabulating successful and
`unsuccessful hits on targets (links or icons). Gross errors
`(unsuccessful hits) for all subjects for the task series during
`which we controlled the delay times are summarized in
`Table 1.
`
`DISCUSSION
`After analyzing the results of the experiment, we arrived at
`the following conclusions:
`
`• Despite an overwhelming preference expressed for the
`links-first method of selection (15 of the 16 subjects), the
`overall error rate for the two methods of selection (links(cid:173)
`first and icons-first) were very similar.
`
`• The subjects' error rates decreased as they became more
`familiar with a method of interaction.
`
`Ill Links-first, 2/5 sec
`•
`Links-first, 4/5 sec
`Iii Icons-first, 2/5 sec
`l111cons-first, 4/5 sec
`
`Task 1 Task 2 Task 3 Task 4 Task 5 Task 6 Task 7 Task 8
`
`Figure 4. Line graph of data from Table 2
`
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`PAPERS
`
`• Subjects had greater difficulty selecting links in icons(cid:173)
`first mode, and vice versa. This accounts for the
`alternating peaks and troughs of Figure 4.
`
`• In the questionnaire, we asked subjects how they would
`change the behavior of the delayed response. Many
`expressed the same desire: that all objects in one layer
`that did not overlap with objects in the other layer should
`be immediately responsive- in other words, that the
`subject should not have to wait for the mouseclick to pass
`through to the lower level if there is nothing on top of it
`in the upper level. This result surprised us, as we
`expected that a rigorous and consistent distinction
`between the layers would make it easier for subjects to
`conceptualize the two layers.
`
`" The request for immediate responsiveness was given
`added weight by the subjects' answer to another
`question. When asked which delay time they preferred,
`most (12 of 16) responded that they preferred a delay
`time of 1/5 second- the minimum allowed by the
`prototype, regardless of which selection method they
`used. What the subjects appeared to be asking for was a
`way to simulate immediate responsiveness of non(cid:173)
`intersecting objects, within the constraints of the
`prototype.
`• The request was further supported by the difficulties that
`all subjects had with selection of non-obscured text while
`using the icons-first selection method. This may account
`for the high error rate in Task 6, when performed in
`icons-first, 2/5 second mode (see Figure 4). Users
`performing the same task in icons-first, 4/5 second mode
`had less trouble, possibly because they were by Task 6
`accustomed to holding down the mousebutton for the
`extended duration required to select a text link.
`
`DIRECTIONS FOR FUTURE WORK
`This expressed preference for immediate responsiveness of
`non-intersecting objects merits additional research. How, for
`example, would this change to the delayed-response model
`affect complex text selection, such as the drag-select gesture
`supported by many PDAs? How would it affect selection of
`objects in PDA-based drawing applications? Would novices
`find this inconsistency in the method of object selection
`confusing? Would further experiments support the subjects'
`intuition that immediate responsiveness is a more efficient
`way to select non-intersecting objects, and might the data
`from those experiments reveal differences between the links(cid:173)
`first and icons-first methods?
`
`CONCLUSION
`The very small screens of PDAs severely limit the space
`available for the display of both text and the widgets
`required to navigate within or modify that text. A promisin.g
`method for maximizing the usable screen space of PDAs IS
`to vary the transparency of overlapping objects on the
`screen, so that objects beneath other objects are still v~sible,
`and valuable screen space is not sacrificed in order to display
`both widgets and information at the same time. In this paper,
`we have proposed that a variable delay in the response of
`
`overlapping widget and text improves the effectiveness of
`the semi-transparent widget/text model. We assumed that
`varying the delay by which objects in different virtual layers
`of the screen responded to users' attempts to select those
`objects would make it possible for users to select partially(cid:173)
`obscured objects without having to resort to a toggle to
`switch between layers.
`
`Our experiment with a prototype for an online newspaper
`that uses this selection technique bears out this assumption,
`and raises additional questions. After an initial learning
`period, the test subjects were able to select underlying screen
`objects, regardless of whether those objects were text or
`icons. Subjects did, however, have greater difficulty
`successfully selecting objects of one kind when the other
`kind of object was on the top layer. Our results suggest that it
`may be easier for subjects to use a variant of the semi(cid:173)
`transparency/delayed response model, in which the delay in
`responsiveness does not apply to objects in one layer which
`do not appear to intersect with objects in the other layer.
`
`Nothing in our experiment calls into question the merits of
`techniques for maximizing usable screen space that rely on
`alternative views of information content. Indeed, it should be
`possible to combine this semi-