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  • Standardized Approaches to Usability - ISO 9241

    Posted on April 13th, 2009 chris No comments
    Usability is an essential part of every project, and implementing it as part of the design and development process involves the systematic identification of requirements for usability. In recent years a number of standardized approaches have been established, with contributing norms and standards such as ISO 9241 and 9126. In this post we are having a look into the ISO norm 9241 which has the title "Ergonomics of Human System Interaction". It is a multi-part standard, divided into 28 portions, with part 11 being "Guidance on usability" (pdf).
    "Usability is an important consideration in the design of products because it is concerned with the extent to which the users of products are able to work effectively, efficiently and with satisfaction."
    Let's start with the norm's definitions:
    • Usability: 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.
    The three key elements are:
    • 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
    These are very general definitions and need to be identified for each project individually. The ideas behind ISO 9241 are meant to assist this process with a systematic approach for identification and evaluation of usability-objectives.

    How to measure Usability?

    In order to specify or measure usability it is necessary to identify the goals and to decompose effectiveness, efficiency and satisfaction and the components of the context of use into sub-components with measurable and verifiable attributes.
    In short, a project needs:
    • Clearly described (usability-)goals and intentions
    • Description of the context of use (including users, tasks, equipment and environments)
    • Target and current values of intended effectiveness, efficiency and satisfaction
    The norm includes the following figure visualizing the "usability framework", showing different components of usability and their relations: usability-framework2

    Goals

    The goals of use should be described as accurate as possible and divided into sub-goals. Each sub-goal is a component of an overall goal and has a certain criteria which would satisfy this goal.

    Context of use

    Each of the elements of the context of use needs to be described as detailed and accurate as possible:
    • Users by relevant characteristics, including knowledge, skill, education, experience, training, motor and sensory capabilities, etc. It may also be relevant to divide them into groups of users (eg. to different levels of experience).
    • Tasks are all activities to achieve a goal, and especially those tasks which may influence usability need to be identified and described. It should not be described solely in terms of functions or features, but should be related to the goals (such as the goals for usability)
    • Equipment includes relevant characteristics of the hardware, software and materials associated with the interface
    • Environments should be described from as many perspectives as possible, especially the physical and social elements. These may include physical factors such as workspace, furniture, temperature and humidity as well as social factors such as work practices, organizational structures and attitudes
    Content of use description checklist

    Metrics for Usability

    It's necessary to identify at least one measure for each of effectiveness, efficiency and satisfaction which are corresponding with the goals. These are highly individual for each project and there is no general rule how measures should be chosen or combined. It's also important to decide about the level of detail for each measure. For example where usage is infrequent, high importance may be given to measures of learning and re-learning.
    • Effectiveness: This measures the goals of the user to the accuracy and completeness with which these goals can be achieved.

      If the desired goal is to accurately reproduce a two-page document in a specified format, then accuracy could be specified or measured by the number of spelling mistakes and the number of deviations from the specified format, and completeness by the number of words of the document transcribed divided by the number of words in the source document.

    • Efficiency: This measure relates the level of effectiveness achieved to the expenditure of resources. Relevant resources can include mental or physical effort, time, materials or financial cost.

      If the desired goal is to print copies of a report, then efficiency could be specified or measured by the number of usable copies of the report printed, divided by the resources spent on the task such as labour hours, process expense and materials consumed.

    • Satisfaction: Satisfaction measures the extent to which users are free from discomfort, and their attitudes towards the use of the product.

      Satisfaction can be specified and measured by subjective rating on scales such as discomfort experienced, liking for the product, satisfaction with product use, or acceptability of the workload when carrying out different tasks, or the extent to which particular usability objectives (such as efficiency or learnability) have been met. Other measures of satisfaction might include the number of positive and negative comments recorded during use.

    Overall Usability

    Selecting just the most important user-goals is likely to be ignoring many functions, but has proven an efficient practical approach. The goals (usability objectives) can be overall goals (eg. produce a letter) or narrowed down goals (eg. perform search and replace), and overall usability is composed of the individual measures of effectiveness, efficiency and satisfaction: overall_usability1 Like this it is relative easy to create an overview of the usability objectives and their respective measures for effectiveness, efficiency and satisfaction. At various stages during the development process, the developers can measure the usability achieved against these targets. A checklist for the usability objectives might look like this:
    (ISO 9241 part 11, p. 17)
    The ideas behind ISO 9241 structured in a list like that makes it easier to systematically identify, collect and review usability objectives and their respective levels of completion, finally helping to provide a better product to users as well as a standardized approach to usability for developers. * If you have any suggestions, ideas or feedback, please leave a comment below! Interesting resources include ISO9241 Part 11, [[Usability]] and The Art of Unix Usability, amongst many others! Next posts in this series include the ISO 9126 norm and a practical step-by-step example.

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