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Conceptual Framework and Models for Identifying and
Organizing Usability Impact Factors of Mobile Phones
Dong-Han Ham
1
, Jeongyun Heo
2
, Peter Fossick
3
, William Wong
1
, Sanghyun Park
2
,
Chiwon Song
2
, Mike Bradley
3
1
School of Computing Science
Middlesex University
The Burroughs London
NW4 4BT UK
{d.ham, w.wong}@mdx.ac.uk
2
MC R&D Centre
LG Electronics
Seoul Korea
{jy_heo, sanghyun, chiwon79}
@lge.com
3
Product Design Engineering
Middlesex University
Bramley Road London
N14 4YZ UK
{p.fossick, m.d.bradley}
@mdx.ac.uk

ABSTRACT
Usability has been regarded as a critical factor affecting
the quality of mobile phones. Many studies have
examined usability impact factors of mobile phones on
the basis of software usability concepts. However,
considering mobile phones as multi-media and
information appliances, a new usability concept and
associated factors should be developed. This paper
proposes a conceptual framework which has five views to
reflect different aspect of interactions between users and
mobile phones, and from which various usability impact
factor models can be derived. Five views include user
view, product view, interaction view, dynamic view, and
execution view. Furthermore, we developed a hierarchical
model which organizes usability factors in terms of goal-
means relations. Through two case studies, we could
verify the usefulness of the framework and model. Lastly,
we developed a set of checklists that are helpful to
measure the usability of mobile phones, thereby
increasing the practicality of the framework and model.
Author Keywords
Usability, Mobile user interfaces, Interface evaluation
ACM Classification Keywords
H1.2. Models and principles: User/machine systems.
INTRODUCTION
It has been reported that usability is one of the most
important attributes affecting the quality of mobile
phones and thus users’ satisfaction (Ketola and Röykkee,
2001). Usability has been defined in various ways, but the
concept of usability defined in ISO/IEC 9126 (1998) is
widely accepted (Hornbaek, 2006). According to the
definition, usability refers to ‘the capability of the
(software) product to be understood, learned, used and be
attractive to the user, when used under specified
conditions.’ Although it is the definition focusing on
software systems, it can be applied to mobile phones in
consideration of features specific to mobile phones.
Usability can be considered both from a design and
evaluation perspective (Folmer et al., 2003). Usability is
one of a range of non-functional requirements, such as
safety and security, which should be satisfied as part of
the design process. Therefore, it should be properly
specified during requirements analysis and designed
during the architectural and implementation design
phases. Conversely, usability is the concept that needs to
be evaluated from a user-centric point of view. User
perception of usability is influenced by many design
factors including visual appeal, hedonic qualities, logical
task sequences, and pleasure in use, as well as contextual
factors including the users’ environment (i.e. context of
use). Thus, it can be said that usability is not an absolute
concept determined by the design activity only, but rather
the relative concept that can be affected by unspecified
factors.
To evaluate usability in a more systematic way, many
studies examined factors or dimensions constituting
usability (Bevan, 1999). For example, ISO/IEC 9241
(1998) defines three dimensions: effectiveness, efficiency,
and satisfaction. Another example is those described in
Nielsen (1993): learnability, efficiency of use,
memorability, errors, and satisfaction. These dimensions
can be classified into two main groups: objective and
subjective dimensions. An objective dimension generally
measures how well users’ tasks are supported by applying
task performance measures like task completion time and
the number of errors. However, objective dimensions do
not always predict the user’s assessment of usability
because it does not reflect users’ feeling or satisfaction.
Subjective dimensions therefore needs to be assessed to
provide a holistic and complete usability measurement.
Usability can be measured in various ways; however, they
can be categorized in three methods: usability testing,
usability inquiry, and usability inspection (Zhang, 2003).
It cannot be said that one method is the best in all
situations. Hence it is necessary to choose an appropriate
method, taking into consideration evaluation purposes,
available time, measures to be collected and so on.
We have described the general concepts concerning
usability so far, without considering the features peculiar
to mobile phones. To examine the usability of mobile
phones, it would be useful to understand the user
interface of mobile phones, tasks to be completed, and the

OZCHI 2006, November 20-24, 2006, Sydney, Australia.
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OZCHI 2006 Proceedings ISBN: 1-59593-545-2

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context of use of mobile phones, therefore it is useful to
briefly describe them in the following section.
Mobile Phones and Tasks
Mobile phones are a typical example of mobile or
handheld devices. These devices are portable, self-
contained information and communication systems, and
are characterized by three features: (1) they are used
primarily in a user’s hands, (2) they are operated without
cables, and (3) they support the addition of new
application and internet connection (Weiss, 2002).
User Interface of Mobile Phones
Figure 1 shows a typical user interface of a mobile phone
(LG, U2050 User Guide). Ketola and Röykkee (2001)
divided user interface elements into seven categories:
input (e.g. softkey, alphanumeric keys, and navigation
tools), display (e.g. icons and indicators), audio and
voices (e.g. ringing tones and microphone), ergonomics
(e.g. touch and feeling, and slide), detachable parts (e.g.
SIM card and battery), communication method (e.g.
Bluetooth), and applications (e.g. making a call, games).
User interface is one of the interaction elements affecting
usability. Other interaction elements include external
interface and service interface. External interface contains
user support, accessories, and supporting software.
Service interface refers to service provider’s services.


Figure 1. User interface of mobile phones.
The seven categories described above are useful to
understand the characteristics of mobile phone user
interface. However, the categories can be generalized into
the three user interface types that are more helpful to
study users’ interactions with mobile phones (Kiljander,
2004). The three types are: logical user interface (LUI);
graphical user interface (GUI); and physical user interface
(PUI). In this paper, LUI (e.g. menu structure and
navigation structure) is defined as interface related to
information contents and layout for task execution. GUI
(e.g. icon and font) is concerned with graphical or visual
items presenting information which users need to perform
tasks. PUI (e.g. keypad and microphone) means tangible
elements supporting physical operation needed for
carrying out tasks.
A key feature and constraint of user interface is that
mobile phones have too little screen to display a lot of
information at the same time; therefore, information
organization and navigation are critical usability issues.
The second significant feature is that a physical button or
key has generally more than one control function. Thus
the functions of a single key are dependent on types of
modes. The third is that processing power and available
memory are limited.
Interaction with Mobile Phones
As mobile phones have increasing number of functions,
users can do various tasks using mobile phones. The most
frequently used functions include making a call, sending a
message, personal information management, listening
MP3 files, changing settings, and taking pictures, playing
games. Other functions are watching TV, remote control
of systems on home automation, internet banking,
personal computer functions, etc. In the forthcoming
ubiquitous computing environment together with the
continuing patern of convergence mobile phones are
expected to play a key role accessing a wide range of
services and will therefore be an essential device within
contemporary life.
The usability of mobile phones needs to be evaluated
taking account of their context of use. Certainly, user
interface elements are important factors affecting the
usability of mobile phones. However, usability is also
influenced by other factors from user groups and their
preferences, task types, social constraints and so on.
Citing the terminology from the area of software quality,
we need to focus on quality-in-use rather than external or
internal quality (Bevan, 1999). For this reason, usability
impact factors should be examined with systematic
viewpoints addressing all the aspects concerned with the
interaction between users and mobile phones.
Research Problems
To date, several studies have examined various kinds of
factors affecting the usability of mobile phones.
Examples are: efficiency; object layout; accessibility;
consistency; screen size; navigation; battery life;
feedback messages; user guidance; naming/labelling; etc.
Such studies helped us understand what could constitute
usability impact factors; however, very few attempts have
been made at developing a conceptual framework and
impact factor model for organizing the factors, which
usability engineers actually need. Additionally, most of
studies concerned with usability of mobile phones have
applied software usability concepts originating from the
study of static personal computers (PCs). Although these