Conceptual Framework and Models for Identifying and Organizing Usability Impact Factors of Mobile Phones.
Dong-Han Ham1, Jeongyun Heo2, Peter Fossick3, William Wong1, Sanghyun Park2,
Chiwon Song2, Mike Bradley3
1School of Computing Science, Middlesex University, The Burroughs London, NW4 4BT UK. {d.ham, w.wong}@mdx.ac.uk
2MC R&D Centre, LG Electronics, Seoul Korea. {jy_heo, sanghyun, chiwon79}@lge.com
3Product Design Engineering, Middlesex University, Bramley Road London, N14 4YZ UK. {p.fossick, m.d.bradley}@mdx.ac.uk
OZCHI 2006, November 20-24, 2006, Sydney, Australia.
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 goalmeans 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.
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 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).
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.
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 context of use of mobile phones.
User Interface of Mobile Phones
Ketola and Röykkee (2001) divided user interface elements into seven categories:
1 input (e.g. softkey, alphanumeric keys, and navigation tools),
2 display (e.g. icons and indicators),
3 audio and voices (e.g. ringing tones and microphone),
4 ergonomics (e.g. touch and feeling, and slide),
5 detachable parts (e.g. SIM card and battery),
6 communication method (e.g. Bluetooth), and
7 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.
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:
1 logical user interface (LUI);
2 graphical user interface (GUI); and
3 physical user interface (PUI).
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.
My Comments: Usability problems of mobile phones are:
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.
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
…..
Conceptual Framework
Axioms > Our Approach
Usability is emergent property. > Our position is that usability cannot be absolutely evaluated. Instead several evaluation results based on several viewpoints can indicate it.
Perceived usability has abstraction hierarchy characterized by goal-means relations. > User view reflects this axiom.
Usability is affected by all the things of mobile phones. > Product view reflects this axiom.
Functionality restricts usability and at the same time is one critical factor of usability. > Interaction (task) view reflects this axiom.
Usability changes along time. > Dynamic view reflects this axiom.
Usability is dependent on user groups. > User and Dynamic view reflects this axiom.
Usability impact factors have different meaning to users in terms of their preference or importance. > Execution view reflects this axiom (quantification scheme).
Designed usability is the outcome of constrained optimization. > Execution view reflects this axiom (weighting value consideration).
Good usability is when perceived usability is greater than expected usability. > Execution view reflects this axiom (a sort of checklist).
Most of critical usability problems can be found in analytical evaluation without actual observation. > Execution view reflects this axiom (a sort of checklist).
Usability has a meaning only under specified context of use. > Evaluation results using a set of checklist should be interpreted with context of use given.
Table 1. Axioms and our approach.
Linking Different Groups of Usability Impact Factors
Three views of the framework indicate that usability impact factors can be categorized into three groups.
The first group is human perceived usability (user view) where typical examples include effectiveness, efficiency, and memorability.
The second is property exhibited by mobile phones (product view) and examples are reliability, durability, performance, and aesthetics.
The third is performance on the tasks (interaction view) and examples are task supportability and error prevention.
Dong-Han Ham1, Jeongyun Heo2, Peter Fossick3, William Wong1, Sanghyun Park2,
Chiwon Song2, Mike Bradley3
1School of Computing Science, Middlesex University, The Burroughs London, NW4 4BT UK. {d.ham, w.wong}@mdx.ac.uk
2MC R&D Centre, LG Electronics, Seoul Korea. {jy_heo, sanghyun, chiwon79}@lge.com
3Product Design Engineering, Middlesex University, Bramley Road London, N14 4YZ UK. {p.fossick, m.d.bradley}@mdx.ac.uk
OZCHI 2006, November 20-24, 2006, Sydney, Australia.
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 goalmeans 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.
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 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).
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.
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 context of use of mobile phones.
User Interface of Mobile Phones
Ketola and Röykkee (2001) divided user interface elements into seven categories:
1 input (e.g. softkey, alphanumeric keys, and navigation tools),
2 display (e.g. icons and indicators),
3 audio and voices (e.g. ringing tones and microphone),
4 ergonomics (e.g. touch and feeling, and slide),
5 detachable parts (e.g. SIM card and battery),
6 communication method (e.g. Bluetooth), and
7 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.
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:
1 logical user interface (LUI);
2 graphical user interface (GUI); and
3 physical user interface (PUI).
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.
My Comments: Usability problems of mobile phones are:
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.
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
…..
Conceptual Framework
Axioms > Our Approach
Usability is emergent property. > Our position is that usability cannot be absolutely evaluated. Instead several evaluation results based on several viewpoints can indicate it.
Perceived usability has abstraction hierarchy characterized by goal-means relations. > User view reflects this axiom.
Usability is affected by all the things of mobile phones. > Product view reflects this axiom.
Functionality restricts usability and at the same time is one critical factor of usability. > Interaction (task) view reflects this axiom.
Usability changes along time. > Dynamic view reflects this axiom.
Usability is dependent on user groups. > User and Dynamic view reflects this axiom.
Usability impact factors have different meaning to users in terms of their preference or importance. > Execution view reflects this axiom (quantification scheme).
Designed usability is the outcome of constrained optimization. > Execution view reflects this axiom (weighting value consideration).
Good usability is when perceived usability is greater than expected usability. > Execution view reflects this axiom (a sort of checklist).
Most of critical usability problems can be found in analytical evaluation without actual observation. > Execution view reflects this axiom (a sort of checklist).
Usability has a meaning only under specified context of use. > Evaluation results using a set of checklist should be interpreted with context of use given.
Table 1. Axioms and our approach.
Linking Different Groups of Usability Impact Factors
Three views of the framework indicate that usability impact factors can be categorized into three groups.
The first group is human perceived usability (user view) where typical examples include effectiveness, efficiency, and memorability.
The second is property exhibited by mobile phones (product view) and examples are reliability, durability, performance, and aesthetics.
The third is performance on the tasks (interaction view) and examples are task supportability and error prevention.
Our approach is hinted by software measurement process framework (ISO/IEC 15939, 2001) and abstraction hierarchy which is popular concept in the area of cognitive systems engineering.
Usability can be interpreted by using usability indicators. Usability indicators can be obtained by the functional combination of usability criteria. Usability criteria can be measured by applying measurement methods to the properties of mobile phones. These properties have specific value as usability data. Figure 4 explains five abstraction levels of usability impact factors.
1 Usability (Quality in Use)
2 Usability Indicator
3 Usability Criteria
4 Usability Property (Metric)
5 Usability Data.
Usability Indicator
While usability cannot be accurately and fully evaluated in any way, it can be estimated or evaluated by some usability indicators which provide a basis for decision
making. On the basis of the literature we reviewed, we concluded that usability of mobile phones can be estimated by five indicators, which are further divided into two dimensions. The five indicators are:
1 effectiveness,
2 efficiency,
3 learnability,
4 satisfaction, and
5 customization.
5 customization.
Of those, the first three indicators are related to task performance dimensions and they are easy to quantify. But the later two indicators, related to emotion human factors, are not easy to quantify.
Usability Property
At the level of usability property, we measure observable features of mobile phones, which have an influence on usability, by directly observing mobile phones or additionally applying some measuring functions (see Figure 6). Usability data below the level of usability
property offer actual value for usability property.
Usability Criteria
As shown in Figure 7, usability criteria evaluate how well usability property is designed to enhance the usability of mobile phone in consideration of task scenarios. Thus
usability criteria are aimed to offer usability-related information that is useful to assess how actual, observed features of mobile phones contribute to usability
indicator.
For example, we can measure ‘minimalism’, which can highly influence on ‘effectiveness’ and
‘efficiency’, by considering softkey mapping, menu contents, and scrolling method.
Overall Hierachy
‘efficiency’, by considering softkey mapping, menu contents, and scrolling method.
Overall Hierachy
The proposed model is similar to the models proposed by Doyanee et al. (2002) and Bosch and Folmer (2004).
But the main difference between those models and our model is that they focused on software, not mobile phones.
CONCLUSIONS
There are a lot of factors affecting usability of mobile phones. Usability needs to be designed and evaluated, taking account of all factors in a unified way. This paper proposed a conceptual framework for identifying and organizing usability impact factors of mobile phones. The framework provides five views which represent different aspects of interaction between users and mobile phones. They include user view, product view, interaction view, dynamic view, and execution view. The framework intends to be a basis from which various usability impact factor models can be developed.
Based on the framework, we developed a goal-means hierarchical model of usability impact factors. In this model, usability impact factors that have different abstraction levels are organized in terms of goal-means relations. This model also classifies factors by design areas, such as LUI and PUI. This hierarchical model can effectively be used for several purposes, such as interpreting usability evaluation items and diagnosing the causes of usability problems. Additionally, we developed an extended hierarchical model, paying particular attention to dynamic aspect of usability and different user groups.
In order to verify the proposed framework and hierarchical model and to obtain information for
improving them, we conducted two case studies. These studies supported the usefulness of the proposed framework and model. To support the use of the hierarchical model, we developed a set of checklist evaluating usability factors by design areas under a task scenario. Furthermore, though we did not describe in detail in this paper, we developed a quantification scheme in this study, which gives a usability score of a mobile phone, applying various weighting value assignment methods. Thus the scheme takes into consideration users’ preference of usability factors.
In order to verify the proposed framework and hierarchical model and to obtain information for
improving them, we conducted two case studies. These studies supported the usefulness of the proposed framework and model. To support the use of the hierarchical model, we developed a set of checklist evaluating usability factors by design areas under a task scenario. Furthermore, though we did not describe in detail in this paper, we developed a quantification scheme in this study, which gives a usability score of a mobile phone, applying various weighting value assignment methods. Thus the scheme takes into consideration users’ preference of usability factors.
Reference that I may want to read further in future:
Donyaee,, M, Seffah, A., Kline, R., and Striva, J. An integrated framework for usability measurement. In Proc. 12th International Conference on Software Quality (2002), 1-10.
Hornbaek, K. Current practice in measuring usability: challenges to usability study and research. International Journal of Human-Computer Studies 64, 2 (2006), 79-102.
ISO/IEC 15939. Software engineering-Software measurement process framework (2001).
ISO/IEC 9126-1. Software engineering-Software product quality-Part 1: quality model (1998).
ISO/IEC 9241. Ergonomic requirements for office work with visual display terminal-Part 11: guidance on usability (1998).
Ketola, P. and Röykkee, M. The three facets of usability in mobile handsets. In Proc. CHI 2001 Workshop (2001).
Kiljander, H. Evolution and usability of mobile phone interaction styles. Unpublished Ph.D. Thesis at Helsinki University of Technology (2004).
Rasmussen, J. The role of hierarchical knowledge representation in decision making and system
management. IEEE Transactions on Systems, Man, and Cybernetics 15 (1985), 234-243.
Weiss, S. Handheld Usability. John-Wiley & Sons (2002).
Zhang, Z. Overview of usability evaluation methods. http://www.usabilityhome.com/ (2003)
Hornbaek, K. Current practice in measuring usability: challenges to usability study and research. International Journal of Human-Computer Studies 64, 2 (2006), 79-102.
ISO/IEC 15939. Software engineering-Software measurement process framework (2001).
ISO/IEC 9126-1. Software engineering-Software product quality-Part 1: quality model (1998).
ISO/IEC 9241. Ergonomic requirements for office work with visual display terminal-Part 11: guidance on usability (1998).
Ketola, P. and Röykkee, M. The three facets of usability in mobile handsets. In Proc. CHI 2001 Workshop (2001).
Kiljander, H. Evolution and usability of mobile phone interaction styles. Unpublished Ph.D. Thesis at Helsinki University of Technology (2004).
Rasmussen, J. The role of hierarchical knowledge representation in decision making and system
management. IEEE Transactions on Systems, Man, and Cybernetics 15 (1985), 234-243.
Weiss, S. Handheld Usability. John-Wiley & Sons (2002).
Zhang, Z. Overview of usability evaluation methods. http://www.usabilityhome.com/ (2003)
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