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Insights from Human Factors International
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In This Issue:
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Breadth vs. Depth
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Kath Straub, Ph.D., CUA, Chief Scientist of HFI, and Susan Weinschenk,
Ph.D., CUA, Chief of Technical Staff for HFI, revisit the issue of Web
site breadth vs. depth.
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The Pragmatic Ergonomist
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Dr. Eric Schaffer, Ph.D., CPE, founder and CEO of HFI offers practical
advice.
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An update on breadth vs. depth
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During the last 5 years a controversy has been brewing concerning the
breadth vs. depth in menu design for Web sites. Which is best? A site
that is broad and shallow, presenting a lot of choices to the user right
away, but only requiring a few layers? Or is it best to have narrow and
deep, which means presenting only a few choices at a time, but requiring
many layers in? As is usually the case, the answer turns out not to be
so simple. In this issue of the newsletter we explore the variables that
are emerging as important in the debate.
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The history of breadth vs. depth
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In the March 1999 issue of this newsletter,
Bob Bailey concluded that breadth was better than depth. He reported on
two studies that also surveyed past literature and concluded that it was
better to have lots of categories in Web menus at the highest level and
therefore reduce the number of clicks needed to get to the end point.
(Zaphiris and Mtei,1998; Larson and Czerwinski, 1998).
Earlier studies by Snowberry, Parkinson & Sission (1983) demonstrated
that in addition to performance and preference declines, navigational
error rates increased significantly as hypertext depth increased. This
study further demonstrated the value of categorical grouping in shallow
structures, showing that participants had an easier time finding resources
in a 64 item list that was functionally organized than in a list with
random presentation.
Follow-up studies (Kiger, 1984; Jacko & Slavendy 1996; Zaphiris and
Mtei 1997) continued to demonstrate that:
- users found resources faster in broader, shallow sites than in narrow
deep ones, and
- users understood broader, shallow sites better than they understood
narrow deep ones.
However, these studies also began to demonstrate that although shallowness
is a unique and defining parameter for success in learning hierarchies,
sites can also be too shallow.
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More variables surface
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Recent research suggests that there is more to it. When we focus specifically
on people browsing the Internet we change the question slightly. Instead
of just asking "Which menu structure is best?" we really need
to ask, "Which menu structures helps users quickly derive a conceptual
model of the site hierarchy?" Creating an accurate representation
of the structure and organization of the site results in a more successful
and efficient navigation through the resource.
Several factors are thought to influence users' success in learning and
traversing information hierarchies. These do include the breadth/depth
of hierarchy, but additional critical factors are: the transparency of
the category and sub-category labels, qualities of information scent,
relative size of categories, and the shape of the hierarchy.
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Good scent and moderation in everything
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Close review of Snowberry et. al. suggests that an alternative explanation
for participants' success with a shallow design may be that the category
labels were discrete enough to support reliable decision making. It seems
that users conceptually manage broader category sets when the names of
the elements within the set are discrete and easy to compare. Clear category
names provide road signs or 'scent' cues about what lurks behind the link.
Good 'scent,' in turn, supports successful category selection. Larson
and Czerwinski (1998) tested the hypothesis that there is an interaction
between structure, scent and category soundness by measuring speed, click-stream
accuracy and perceived 'lostness' while varying breadth/depth parameters
of presentation in a well defined information space. They observed that
the distinctness of category names was particularly helpful at the highest
levels of the information hierarchy, since selecting an incorrect path
at the first hierarchical level often resulted in multiple-click backtracks.
Note, however, that even with clear and distinct category labels, Larson,
et. al. concluded that moderate breadth affords optimal user performance.
The claim that moderate depth supports optimized performance is further
buttressed by Zaphiris (2001) computational models of user performance
in menu search. Using behavioral values from previous HCI studies, Zaphiris'
model predicts that menu design on either extreme (very deep or very broad)
will undermine learnability and usability for users. This is postulated
to be particularly so for older individuals — the fastest growing
segment of Internet users.
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Initial and final selection menus should be broadest
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An additional parameter of site structure design that is currently drawing
more attention is site shape. Consensus is that the initial age should
balance breadth with layout/white space to offer a moderate selection
of navigational options. Work on the optimal shape of a hierarchical site
suggests that concave designs are optimal. A concave shape presents a
broad initial selection screen, followed by category decisions over small
categories and then followed with a terminal option set that is again
somewhat broad. Norman and Chin (1988), and more recently Bernard (2002),
demonstrate that for browse-oriented tasks, concave designs take users
less time to navigate, and evoke less wasted clicks. In contrast, they
observed no differences in ability to successfully navigate the various
menu structures for explicit, target-specific scenarios.
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What now?
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Research comparing navigation efficiency through sites of varying depths
and breadths broadly converges on the findings that users find roughly
16 (ungrouped) top-level links leading into 2-3 subsequent menus the most
efficient, learnable and least error prone. This knowledge is well and
good, but what does that really mean for designers? Today? Now?
First, we can derive some broad (and largely intuitive) design guidelines
from this work:
- Too deep is too deep: users have a more difficult
time encoding, and consequently navigating, deep sites.
- Too broad is too broad: conversely extremely broad
sites (which may encourage satisficing) also present a challenge to
efficient navigation.
- Effective sub-grouping reduces perceived breadth:
grouping navigation elements thematically improves performance for even
the broadest structures.
- Clear labels improve navigation accuracy: creating
clear and distinct labels for navigation elements enhances performance.
More interestingly, careful consideration of the specific tasks used
in these studies suggest that the breadth/depth findings map directly
to effective Intranet design. Consider the typical participant's
task in these studies: find an explicitly named target (search navigation)
or navigate to an implicit, user-selected token within a specified category
(browse navigation). Since Internet users frequently come to the Web looking
for information about a concept, but without a specific page in mind,
it is commendable that researchers have begun to focus more extensively
on browse-driven exploration. However, it also must be noted that both
implicit and explicit search tasks are still essentially serial tasks.
That is, the research participants in these studies completed well-defined,
single tasks, returning to the home page before initiating
the subsequent task. This approach maps directly to the serial task completion
behavior patterns observed for the frequently executed Intranet tasks:
find a phone number; download a form; check the stock price; change personal
benefits information, find a policy.
Thus, the breadth/depth research speaks directly to optimal structure
for Intranet design. Further, the cumulative findings of the research
challenges the widely implemented approach that Intranets should focus
corporate or institutional news wrapped in what is typically a tab-based
(or tab plus left side navigation) functional navigation design. Instead,
this research suggests that moderately broad site structures, consisting
essentially of functionally grouped, transparently labeled link lists
will provide the most effective navigation structure with the best perceived
usability on Intranets. Anecdotal user-centered field analysis and prototype
validation provides additional support for this approach to Intranet design.
Designers who want to know how to take advantage of the depth/breadth
research need to think about the kinds of tasks that people do on their
sites, and how people approach doing those tasks. Do users tend to do
one task at a time? Do they finish that task before they start another?
(Intranets are one example of a site type where this serial task completion
model holds.) If so, then the task approach on your site is parallel to
the task flow that was tested in the depth/breadth work. Therefore, these
results apply and a broad, shallow menu architecture should provide users
the most efficient and learnable access to resources on your site. In
this case, however, the specific type of site is less important. In applying
these results it is more important to think about what people do, and
how they do it on your site.
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References
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Bernard, M.L. (2002) Examining the effects of hypertext shape on User
Performance. Usability News, 4.2. Original
paper
Jacko, J.A. and Slavendy, G. (1996). Hierarchical Menu Design: breadth,
depth and task complexity. Perceptual and Motor
skills, 82, 1187-1201.
Kiger, J.I., (1984). The depth/breadth tradeoff in the design of menu-driven
interfaces. International Journal of Man-Machine
Studies, 20, 201-213.
Larson, K and Czerwinski (1998). Web page design: Implications of memory,
structure and scent from information retrieval.
Proceedings of the Association for Computing Machinery's Computer Human
Interaction Conference, 18-23.
Norman, K. L. and Chin, J. P. (1988). The effect of tree structure on
search performance in a hierarchical menu selection system. Behaviour
and Information Technology, 7, 51-65.
Snowberry, K. Parkinson, S. and Sisson, N. (1983). Computer Display Menus.
Ergonomics, 26, 699-712.
Zaphiris, P. (2001). Age Differences and the Depth-Breadth Tradeoff in
Hierarchical Online Information Systems. In C. Stephanidis (Ed.), Universal
Access in HCI. Mahwah, NJ: Lawrence Erlbaum.
Zaphiris, P. and Mtei, l. (1997). Depth v. Breadth in the Arrangement
of Web Links. Original
paper
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We looked at this issue before in the March,
1999 newsletter... Keep using hierarchical menus, with 18-24 overall
choioces, grouped. Have up to 10 choices in a group.
NOW, we see it is really important that the top-level group headers stand
out and create a crisp overall picture of the site structure. Good point.
Reducing the user's feeling of 'lostness' can mean increased sales, increased
Intranet usage, increased likelihood of buying an application, and other
delightful outcoms.
It is fascinating to see research on site structure move beyond just
the home page design. We need to look at the overall flow. I am far from
convinced that a concave structure is always best. But it is good to think
deeper when we design structures.
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Past Issues
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