Usability refers to how someone using a piece of software evaluates its quality of use for a particular tasks and in a particular context.
A number of definitions have been proposed in the literature that emphasize that the quality of use cannot be separated from its utility. So these definitions would not allow to state that something has usability but is not useful (usable, but not useful).:
- Bevan (1995:4): "Quality of use: the extent to which a product satisfies stated and implied needs when used under stated conditions."
- the former has been the basis for the definition in the ISO Standard (ISO 9241) on usability from 1998: "Extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use."
- Holzinger (2012:71): "Usability is most often defined as the ease of use and acceptability of a system for a particular class of users carrying out specific tasks in a specific environment. Ease of use affects the users’ performance and their satisfaction, while acceptability affects whether the product is used."
In contrast, another widely used definition by Nielsen (2012) would separate usability from utility. According to him, usability only refers to the ease of use and how pleasant the user experience is, while utility describes whether the features provide are of use to someone using the software. Nielsen talks of usefulness if both usability and utility are present.
A useful overview about the development of the different concepts of usability see Cockton (2014) - Usability Evaluation in the The Encyclopedia of Human-Computer Interaction.
Theories and Results
- usability is quality of use for an intended purpose
- quality of use is not product-immanent measure that is absolute. Instead, it is dependent on the evaluation of users and their respective use case (i.e. purpose of use) and hence might vary from one user to another
- the evaluations of users depends on the context of this use (the technology, the physical environment as well as the social and organisational environment)
There exist now various standards that define usability and related concepts as well as methods for measuring it. For an overview of the different relevant standards see Earthy et al. (2012) - ISO Standards for User-Centered Design and the Specification of Usability. One example is ISO Standard (ISO 9241) which breaks down the concept of usability in three components (see ISO 9241-11 Part 3):
- effectiveness: "Accuracy and completeness with which users achieve specified goals."
- efficiency: "Resources expended in relation to the accuracy and completeness with which users achieve goals."
- satisfaction: "Freedom from discomfort, and positive attitudes towards the use of the product."
This standard is currently being updated (see Bevan et al. (2015)).
It also features research from Psychology.
It has been widened to include User Experience and User Centered Design.
Methods & Measurement
The usability of a system is measured both in order to evaluate an existing application and in order to guide the development of an application. There exist a number of different methods to measure usability. Holzinger (2012) distinguishes between:
- inspection methods - need no actual (end) users of the system, measuring against a set of criteria such as availability of help/documentation, language, consistency ...
- heuristic evaluation - evaluators check system against a set of certain criteria
- cognitive walkthrough - defines particular tasks that need to be accomplished and checks those against certain criteria
- action analysis - measuring the interaction of an evaluator with the website
- test methods - involves actual users of the system
- thinking aloud - while using the site a user comments on activities
- field observation - an evaluator observes the use of actual users, this includes log data (e.g. web logs)
- questionnaires - surveying users, a testing method that does not require the user to actively use the site at the point of testing
For the evaluation of the usability of software and system exist a wide variety of standardised questionnaires that differ in what dimensions they cover and how many question items they use. Examples include (from Christophersen and Konradt (2011):
- PUTQ by Lin et al 1997 for software
- AttrakDiff by Hassenzahl 2010 for interactive products
- WAMMI by Kirakowski et al. 1998 for websites
- UFOS by Konradt et al. 2003 for online stores
- QUIS by Chin et al (1998)
- IsoMetrics by Gediga et al. (1999) with 90 different items
- PSSUQ by Lewis (2002)
- SUS by Brooke (1996)
- ASQ (After-Scenario-Questionnaire) by Lewis (1991:79) with 3 questions
- "Overall, I am satisfied with the ease of completing the tasks in this scenario."
- "Overall, I am satisfied with the amount of time it took to complete the tasks in this scenario."
- "Overall, I am satisfied with the support information (on-line help, messages, documentation) when completing the tasks?"
- SIUM (single-item usability measure) by Christophersen and Konradt (2011) with just one question
- "Overall, I am satisfied with the usability of this store."
See usabilitynet.org for links to questionnaire resources.
Pages in category "Usability"
The following 11 pages are in this category, out of 11 total.