0018-8670/99/$5.00  (C)  1999 IBM

Multistream input: An experimental study of document scrolling methods 

by S. Zhai and B. A. Smith 

Navigating through on-line documents has become an increasingly common task in 
human- computer interaction. This paper investigates alternative methods to 
improve user performance for browsing World Wide Web and other documents. In a 
task that involved both scrolling and pointing, we compared three input methods 
against the status quo. The results showed that a mouse with a finger wheel did 
not improve user's performance; two other methods, namely a mouse with an 
isometric rate-control joystick operated by the same hand and a two-handed system 
that put a mouse in the dominant hand and a joystick in the other, both 
significantly improved users' performance. A human factors analysis of each of 
the three input methods is presented. 

The WIMP (windows, icons, menus, and pointer) interaction style continues to gain 
an increasingly wide range of applications, despite its age and long history.[1] 
The rapidly developing World Wide Web (WWW) makes the use of this style of 
interaction even more frequent and intense. As a result, the limitations of 
existing WIMP features also become more severe and obvious. There have been 
numerous interface inventions and studies since the basic WIMP style was 
developed (e.g., see Reference 2), but they have been largely restricted to the 
research literature and isolated demonstrations. Both the unavailability of new 
commercial technology (hardware and software) and an incomplete understanding of 
human factors have contributed to the lack of major improvements in the 
mainstream interfaces. 

One basic feature of the existing mainstream WIMP interfaces is that the user 
communicates with the computer system via a single stream of input. Such input is 
physically driven by a two-degree-of-freedom input device, typically a mouse, and 
graphically displayed as a cursor. Depending on the cursor position, such as on a 
document, a window, a menu, a scroll bar, an icon, or a hyperlink, the function 
of the cursor switches from pointing, to selection, to drawing, to scrolling, to 
opening, and so on. Such a single-stream operation, needless to say, has offered 
users many advantages such as ease of understanding and learning the interaction 
mechanism. The disadvantage, however, is the limited communication bandwidth[2] 
and the costs in time and effort of acquiring widgets and control points.[3,4] A 
particular case is document browsing, which is one of the most frequent tasks in 
interacting with computers. A document, such as a text file, a spreadsheet, a 
folder, and most importantly, a WWW page, is often larger than the viewing 
window. Only a portion of the document can be displayed at a time. The user often 
has to move (scroll) the window to view other portions of the document. Scrolling 
is traditionally done by dragging the scroll bar handle on the side or the bottom 
of the viewing window. There are at least the following three limitations to such 
a method: 

1. It takes a certain amount of time, T1, to acquire (point to and press on) the 
scroll bar. According to the well-studied Fitts' law,[5] T1 is logarithmically 
proportional to the ratio of A and W. A is the distance the cursor has to travel, 
and W is the size of the widget acquired. At the extreme case (travel across the 
entire screen to acquire the arrow widget at the end of a scroll bar), the Fitts 
index of difficulty can be up to eight bits, which may take more than two seconds 
to complete.[6] 

2. There are three methods of using a scroll bar; each has some limitations. 
First, the user can acquire the moving handle and drag it. The advantage of this 
method is that the user can scroll the document at a controlled speed that is 
suitable for the particular task. The disadvantage is that the dragging function, 
which requires that pressure be maintained on a button while moving the input 
device, is more difficult and takes more time than pointing over the same Fitts' 
index of difficulty.[7] The second method is to use the cursor to press the arrow 
buttons at the ends of the scroll bar, causing the document to scroll at a speed 
that is not adjustable by the user. This is binary control: either move at a 
fixed (or accelerated speed) or stop. The speed could be too slow when the user 
wants to move very far, or too fast when the user wants to visually track the 
document. The third method is to click on the rest of the space on the scroll 
bar, causing the document to "jump." The user is often unaware of the increment 
by which the window jumps, therefore losing track of the target area. 

3. Perhaps most importantly, when the user has to go to the scroll bar to move a 
document, even by just one line, it takes the perceptual, cognitive, and motor 
resources away from the target that the user focused attention on, hence breaking 
the work flow. One basic principle of good user interface design is to help the 
user to focus on the task, not the user interface. Graphical scroll is a case 
that violates this principle. 

The above analysis shows that the standard, single-input stream WIMP interface is 
inadequate for browsing documents. This study looks into three alternative 
methods for browsing. We conducted a human factors analysis on each of the three 
techniques, which was followed by a formal experiment that compared these methods 
against the standard single-stream method. 

Three alternative methods for browsing 

Mouse with isometric joystick. As shown in Figure 1, one alternative device we 
studied is the Joystick Mouse, a two-button mouse with a miniature joystick 
mounted between the two buttons. The mouse retains all usual functions of a 
standard mouse. The miniature isometric joystick, an IBM TrackPoint* pointing 
device, is a rate-controlled input device.[8,9] Each of the two devices can 
function as an independent normal two-degree-of-freedom input device. In the 
current study we assign the mouse for pointing and the miniature joystick for 
scrolling. We hypothesized that the isometric joystick is particularly suitable 
for scrolling tasks based on the following analyses. 

First, let us briefly review the two basic types of transfer functions in input: 
position and rate control (see Poulton[10] for a detailed review). Position 
control, also called zero order control, maps the user input variable to the 
cursor displacement according to a constant or variable gain. Rate control, also 
called first order control, maps the user input variable to cursor velocity. As 
shown in recent six-degree-of-freedom input control studies,[11-13] position 
control is better conducted with isotonic, free-moving devices, such as the 
mouse, and rate control is better conducted with isometric or elastic devices. 
The key factor to this compatibility issue is the self-centering effect in 
isometric or elastic devices. With self-centering, rate control can be easily 
done. Without it, rate control requires conscious effort. Either position control 
or rate control can give users the ability to control all aspects of movement, 
including displacement, movement speed, or higher-order derivatives, but each 
mode corresponds to only one aspect directly: displacement or speed. 

Scrolling, or navigating through a document, requires the user not only to 
control the final displacement of the document to make the target appear in the 
viewing window, but also to control the speed of the movement so that the user 
can comfortably scan the document to look for the target. An isometric rate 
control device apparently meets these requirements. In contrast, if we use an 
isotonic position control device, such as the mouse, the user may not be able to 
control the speed of movement continuously. In particular, due to physical 
constraints (of either the human arm or the mouse pad), position control allows 
the user to move only within a certain distance at one stroke. The user has to 
release (by lifting the mouse) and re-engage the position control device 
repeatedly in order to scroll over a longer distance. 

When using the Joystick Mouse, the user can use either the index finger or the 
middle finger to operate the joystick. In practice, most users use the index 
finger. When using the index finger, the user has to switch the same finger 
between the left button and the joystick. Because of its close proximity, the 
user can rely on kinesthetic memory to locate the stick without looking at it. 

Mouse with finger wheel. The idea of adding an additional sensor to a mouse is 
not new. As described in Venolia,[14] a thumb wheel can be mounted onto a 
standard mouse for an additional degree of freedom in a three-dimensional 
interface. The first commercial mouse with scroll capability, the ProAgio** by 
Mouse Systems (Figure 2), dedicated a wheel to scrolling. We call this class of 
device Wheel Mouse and used a recent model, the Microsoft IntelliMouse** (Figure 
3) in the current study. 

The wheel in the IntelliMouse works in position control mode. It is largely 
free-moving (isotonic) but with a detent mechanism. The control gain from a 
detent step to the number of lines of scrolling is adjustable. To set the control 
gain high will make the scrolling faster. However, in order to be able to scroll 
documents at the resolution of a line of text, the detent step should be set to 
one line. The user may repeatedly stroke the wheel for long movement. 

The IntelliMouse provides two additional modes of scrolling; both turn the mouse 
itself into a rate control device. As analyzed earlier, an isotonic device lacks 
the self-centering effect that is desirable in rate control. In one mode of the 
IntelliMouse, the user presses down on the wheel, which is also a button, to turn 
the mouse body movement to rate control scrolling: the further the mouse is moved 
from where the wheel is pressed, the faster the document scrolls. When the user 
releases the wheel, scrolling stops. In the second mode, the user presses and 
releases the wheel to start the rate control scrolling. Any following click, 
either on the wheel or on other buttons, stops the scrolling. In both cases, a 
visual anchor is displayed on the screen to indicate where the rate control 
scrolling starts. This may help the lack of centering effect in the mouse for 
rate control, but such a centering feedback comes from the visual channel, not 
the haptic feel. 

As with the Joystick Mouse, the user can roll the finger wheel for scrolling with 
either the index finger or the middle finger. Pointing is done by normal mouse 
movement. 

Two-handed joystick and mouse. The third method we studied was a two-handed input 
method. A keyboard with a TrackPoint (between the G, H, B keys, as in IBM 
ThinkPad* computers) and a standard mouse were used in this method as shown in 
Figure 4. The user operates the joystick with a nondominant hand to do scrolling 
and manipulates the mouse with the dominant hand to do pointing. Note that the 
system can be easily set for left-hand-dominant users. 

The idea of using the nondominant hand for a scrolling task has been advocated by 
researchers such as Buxton[2] for over a decade. Scrolling was also one of the 
first scenarios in which two-handed input was experimentally demonstrated to be 
superior to the standard one-handed input. Equipping subjects' nondominant hands 
with two strips of touch-sensitive tablet and their dominant hands with a puck on 
a graphics tablet, Buxton and Myers[3] studied users' performance in a text 
document navigation (jump or scroll) and selection (pointing) task. In that 
experiment the participants used their nondominant hand to jump (one strip that 
was absolute position sensitive) or scroll (another strip that was relative 
movement sensitive) the document and used the dominant hand to select targets. 
With such a two-handed setup, a 15 percent (for expert users) to 25 percent (for 
novice users) performance improvement was measured. 

The present two-handed system studied here differs from previous systems[3,4,15] 
in terms of the physical devices used in two-handed interaction. One of the two 
devices in the system is an isometric rate control joystick (Figure 4). There are 
four potential advantages to including an isometric joystick in a two-handed 
system. 

First, there are individual preferences for different types of devices. Having 
one joystick and one mouse in the system gives the user a choice when they need 
only one device. Second, device performance is task-dependent. A unique advantage 
of in-keyboard isometric joysticks is that the user's fingers do not have to 
leave the keyboard, making mixed typing and pointing tasks much faster.[8] 
Including an isometric joystick in the dual-device system gives the user a choice 
when needed for a particular task. 

Third, an isometric joystick requires less space, or footprint, than any other 
device (mouse, touchpad, or trackball). This advantage is not only important for 
portable computing, but also important for a two-handed desktop environment where 
a keyboard with a mouse has already crowded the workspace. Fourth, as pointed out 
earlier, a rate control technique that is compatible with isometric devices can 
be particularly suitable for scrolling tasks; no repetitive release-re-engage 
problem exists as in position control techniques. 

However, the joystick-mouse two-handed system also poses an unanswered 
theoretical question. With previous two-handed systems that have been 
demonstrated to be advantageous, both hands were engaged in the same isotonic 
position modes, which means consistent or similar motor actions across two hands. 
In the current system, the two hands are engaged in different motor control 
mechanisms: one in isotonic position control and the other in isometric rate 
control. Is such a combination still superior to the standard one-handed input 
system? 

What is also conceptually interesting is the contrast between the two-handed 
system (Figure 4) and the Joystick Mouse (Figure 1). Identical transducers were 
used in the two input methods. The only difference was the location of the 
joystick. In the case of the Joystick Mouse, the joystick was on the mouse and 
was manipulated by the same hand that operates the mouse. In the current case, 
the joystick was in the keyboard and was manipulated by a different hand. In 
other words, we are distributing two streams of input in two ways: one puts both 
streams into one hand and the other separates them to two hands. It is 
interesting to find out how user performance differs between the two methods. 

We should briefly mention a human bimanual action theory: the kinematic chain 
(KC) model of bimanual action.[16] The KC model strongly suggests that the two 
human hands work in a cooperative but asymmetric manner. The nondominant hand, 
like a base link in a chain, tends to take precedence (act first), work on a 
larger but coarse scale, and set the frame of reference. The dominant hand, like 
a terminal link in a chain, tends to act later, work in a smaller but finer 
scale, and operate within the frame of reference. The current two-handed system 
coincides with these characteristics very well: the nondominant side acts first 
(scroll first), sets the frame of reference, and moves at a larger distance (rate 
control). The dominant hand acts later and operates within that frame on a 
smaller scale. This model is also what we do in natural life: hold and move a 
document with our nondominant hand and write within the page with our dominant 
hand. 

The experiment 

Experiment design. We chose to model our experimental task after one of the most 
frequent interaction tasks that today's computer users do: Web browsing. A Web 
page, stored as a local file to avoid transmission delay, was presented to the 
participant. The document contains texts from an IBM computing terminology 
dictionary (Figure 5). A hyperlink is embedded at an unpredictable location in 
each page. The user's task was to scroll the document until he or she found the 
target hyperlink (Figure 5, bottom). Clicking on the target word "Next" would 
bring the participant to the beginning of the next Web page. The target word 
"Next" was displayed in a larger font and different color and was preceded with a 
string of asterisks so it could be easily recognized. Each test of the experiment 
consisted of 10 pages of browsing (scroll and point). The size of the Web pages 
was set so that the scroll handle width was 1.3 cm (so it was not too difficult 
to click and drag for the standard mouse condition; see Figure 5, bottom). The 
Web browser viewing area was 24 cm wide and 15 cm long on a cathode ray tube 
display. Participants were allowed to adjust the positions of mouse, keyboard, 
and display on the desk to suit their own preferences. 

Four interaction methods were tested in the experiment: standard mouse (Mouse), 
mouse with a finger wheel (Wheel Mouse), mouse with joystick (Joystick Mouse), 
and mouse with in-keyboard joystick operated by different hands (Two Hands). Note 
that the pointing mechanism is the same with all four methods: mouse movement by 
the dominant hand. A total of 12 volunteers participated in the experiment. A 
within-subject design was used with a Latin square pattern for order balancing. 
Each of the four methods was presented as the first, second, third, or fourth 
technique tested to an equal number of participants. Each participant performed 
the tests with all four methods. With each method, the participants were first 
given one practice run, during which they were asked to explore all modes (in the 
cases of Mouse and Wheel Mouse) and strategies (aggressive or careful). Although 
encouraged to take as much time as they needed, they all finished the 10 pages of 
practice in less than 10 minutes. The participants were then asked to perform two 
consecutive tests (10 pages each test) as quickly as possible. 

The same 10 Web pages were used for all tests to ensure the same task difficulty 
for all methods tested. None of the participants appeared to realize that these 
were the same pages. If some of them did remember the locations of the targets, 
they may have performed overall faster and hence reduced the differences among 
the four methods. We considered such an effect to be weak and hoped that we could 
still detect meaningful significant differences between the methods. 

Of the 12 participants, all had daily experience with using a mouse; five had 
daily experience with using the in-keyboard isometric joystick; all but one had 
no experience with the three alternative methods; one participant had used the 
three alternative methods once before. 

Trademarks on the devices were covered, and the devices were presented as 
research prototypes in order not to influence participants' opinions on each of 
the methods. After completing all four methods, participants were asked to rate 
each of the four methods on a -3 (terrible) to +3 (great) scale based on their 
experience. 

Results. Figure 6 depicts the mean completion time and 95 percent confidence bars 
in each of the two consecutive tests. A repeated measure variance analysis showed 
that participants' completion time was significantly affected by input method 
(F[sub]3,11[/sub] = 20.3, p < .0001). Although Test 2 was significantly faster 
than Test 1 (F[sub]1,11[/sub] = 12.4, p < .01), such an improvement did not alter 
the relative performance pattern of the input methods (Method X Test 
insignificant: F[sub]3,11[/sub] = 1.1, p = .37). 

With the Mouse condition as the reference, the Joystick Mouse and two-hand 
conditions were 22.4 and 25.5 percent faster, and the Wheel Mouse condition was 
8.7 percent slower than the standard mouse condition. Statistically, the 
difference between Mouse and Wheel Mouse conditions (p = .086) and the difference 
between Joystick Mouse and two-hand conditions (p = .57) were not significant. 
All other pairwise comparisons were significant (p < 0.0001, t-Test). 

Participants' subjective ratings based on their experience were similar to the 
performance measurements (Figure 7) except for the difference between Mouse and 
Wheel Mouse. Participants gave the Wheel Mouse a significantly lower rating than 
the standard mouse (p < .05, t-Test). The Joystick Mouse and two-hand conditions 
were rated significantly higher than the other two methods (p value from .01 to 
.0001), but the difference between the two was not significant (p = .86). 

Discussion 

Wheel Mouse. Surprisingly, although it offered dual-stream input, the Wheel Mouse 
did not perform faster than the standard mouse, despite the fact that with a 
single-stream mouse one has to switch between target selection and manipulating 
the scroll bar. Three participants commented that it was tedious and tiring to 
repeatedly roll the wheel, although this mode was intuitive. Although encouraged 
to explore all three modes in the practice phase, only six participants used the 
two other rate control modes in addition to wheel rolling in the real tests. It 
was felt that the rate control mapping functions in the IntelliMouse could be 
improved. However, we believe the lack of self-centering in the isotonic device 
(mouse) places it at a fundamental disadvantage to do effective rate 
control.[11-13] Alternatively, if the mouse functioned in position control mode 
when the button was pressed, users' performance might have been better. The low 
performance of the Wheel Mouse in this task shows that a dual-stream solution is 
not guaranteed to outperform the status-quo single-stream input. 

Joystick Mouse. Supporting our analyses in the introduction, this dual-stream 
input device outperformed the standard single-stream input significantly. 
Subjective ratings also verified its advantages. Comparatively, although both the 
Joystick Mouse and the Wheel Mouse used one hand to handle two streams of input 
(even with the same fingers), the Joystick Mouse significantly outperformed the 
Wheel Mouse, by a mean magnitude of 29 percent. 

Two Hands. Interestingly, no significant performance or rating difference was 
found between the two-handed system and the Joystick Mouse, even though the two 
streams of input were assigned very differently and the two-handed system 
conforms to the KC theory. Nonetheless, the results showed that an asymmetric 
two-handed design, one hand with isometric rate control and the other hand with 
an isotonic position control, worked well, outperforming the status quo by 25 
percent for the browsing task. Concerns were raised as to whether such a 
two-handed system would work at all and whether the user would confuse the 
functions of the two hands. Clearly this was not the case. Note that although the 
two-handed system did not prove to be superior to the one-handed, dual-stream 
input in the current scrolling-pointing task, the conclusion should not be 
applied to many other tasks, such as tool glass studied by Bier, Stone, Pier, 
Buxton, and DeRose[17] and graphical manipulation studied by Leganchuk, Zhai, and 
Buxton.[4] In fact it is extremely difficult, if not impossible, to use the 
one-handed dual-stream solutions in tasks that require parallel actions, such as 
scaling, translating, and rotating a two-dimensional geometry by controlling two 
vertices.[4] 

For the scrolling-pointing task studied in this experiment, there could exist 
suitable techniques other than the ones investigated. One possible example is to 
use the page up and page down keys for scrolling and the mouse for pointing. The 
main advantage with the page keys is that they move the document by precisely one 
window, which is very desirable in the case of an all-text document. There are 
two disadvantages to this solution. One arises when the user needs to move less 
than a page in order to place a picture, a table, or a subroutine of a program in 
the center of the window. One page might move too far in this case. The arrow 
keys that move one line at a time are less efficient. The second and more 
important disadvantage is that the page up and page down keys are located on the 
right side, the same side as the mouse for all right-handed users and many 
left-handed users who use their right hand for mouse manipulation. This means 
that the users have to take their right hand off the mouse in order to reach the 
page keys and move back to the mouse afterwards. This process requires the visual 
attention of most users, since the page keys are too far from the "home row" 
(ASDFGHJKL) for touch-typing and yet not at the very edge of the keyboard, which 
can be more easily reached. 

A simple improvement that can be made is to add another column of keys, such as 
page up, page down, copy, paste, and delete to the left edge of the keyboard. 
With such an arrangement, the majority of users will be able to keep their left 
hands on these keys and their right hands on the mouse. Such an improvement will 
be complimentary to the advantages of dual-stream mice studied in this 
experiment. When users need to move either exactly one or multiple pages, they 
can simply press the page keys with their left hands. When they need to move at a 
nonfixed interval, they can use the dual-stream mice. We recommend that keyboard 
manufacturers consider such a modification. 

We would also like to point out that the task used in the experiment is 
intentionally an experimental abstraction of a real browsing task so that 
differences in performance of the input system could be measured. Real browsing 
behavior includes reading and other components, which may dilute the performance 
differences we found in this study. 

Conclusions 

Three dual-stream input systems, two single-handed and one two-handed, were 
analyzed and compared in a Web browsing task that required scrolling and 
pointing. Both performance measurement and subjective rating showed that a mouse 
with a joystick all controlled by one hand, or a mouse for one hand and a 
joystick for the other, significantly outperformed the current standard 
single-stream mouse input. However, the mouse with a wheel device did not perform 
any better than the standard mouse. In order to take advantage of additional 
input streams, the types of input devices must be appropriately matched to the 
tasks being performed. In addition to much evidence in the literature, this study 
indicates that it is time to add multistream input to mainstream commercial 
systems, although each step of a new design has to be guided by thorough human 
factors research to avoid mistakes. 

Acknowledgment 

We sincerely thank our colleagues who made significant contributions to the 
project that this study is based on, in particular Ted Selker, Ian May, Bobby 
Lee, Ron Barber, and Kim May. A preliminary version of the work has been 
presented at INTERACT '97. This study has formed the basis for a commercial 
product, the IBM ScrollPoint* mouse, using different hardware and software 
technologies. 

*Trademark or registered trademark of International Business Machines 
Corporation. 

**Trademark or registered trademark of Mouse Systems or Microsoft Corporation. 

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Accepted for publication April 26, 1999. 

Author bios

Shumin Zhai

IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, 
California 95120 (zhai@ almaden.ibm.com). Dr. Zhai is a research staff member at 
the Almaden Research Center where he conducts research and development of 
innovative input devices, interaction techniques, theoretical modeling of 
human-computer interaction, advanced graphical user interfaces, and 
eye-tracking-based next-generation multimodel interaction techniques. In the past 
three years he has led the research and development of the IBM ScrollPoint mouse. 
Dr. Zhai received his Ph.D. degree from the University of Toronto where he worked 
on three-dimensional interfaces and six degree-of-freedom input control. His 
numerous publications can be found at http://www.almaden.ibm.com/cs/people/zhai/. 

Barton A. Smith

IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, 
California 95120 (basmith@almaden.ibm.com). Dr. Smith received his B.S. degree in 
chemistry from the University of Texas at Austin in 1972, and his Ph.D. degree in 
physical chemistry from Harvard University in 1977. After two years at Stanford 
University where he was a National Science Foundation Post-Doctoral Fellow, he 
joined the IBM Research Division in San Jose, California, in 1979. In his 20 
years as a research staff member in IBM Research, he has worked on fundamental 
problems in polymer, materials, and computer science, and on IBM products 
including magnetic disks, optical disks, magnetic tape, typewriter ribbon, 
circuit boards, integrated circuits, multi-chip ceramic circuit modules, optical 
data transmission, displays, and IBM ThinkPads. His most recent work centers on 
understanding how people use information technology and improving the 
human-computer interaction experience. Dr. Smith currently manages the Human 
Interface Research group in the Computer Science Department at the Almaden 
Research Center. For more information see: http://www.almaden.ibm. com/u/basmith. 

Reprint Order No. G321-5710.