The foundations of usability requirements development

The term usability is often misunderstood. On the one hand it is more than just how things look, i.e. more than just the user interface design. The usability is supposed to provide the user with optimal support in achieving their professional goals. It encompasses all interactions with the system or product. On the other hand usability is a little less than the totality of the user experience – the general service and the behavior of the service staff towards the user also make up the part of the total experience. So how do you develop usability requirements and what does that have to do with the users’ perceptions and general human performance capabilities?

Let’s start by defining terms

According to DIN EN ISO 9241, usability describes the “degree to which a product, system or service can be used by a particular user in a particular user context in order to efficiently, effectively and satisfactorily achieve a set of goals.” It is measurable, e.g. as the time a user needs to perform a special task. And as such it is also a marker of quality.

Usability Engineering Usability engineering refers to a variety of approaches and techniques that have the purpose of eliciting requirements in a system and product. It is also concerned with developing and realizing usability concepts. The user is the most important part of usability engineering. Separate usability from the functionality would be ill-advised. That would be like saying: “Try out the features and give us feedback on how things are working. Don’t worry about the screen and or about the number of times you have to click on particular elements. We’ll make it pretty and user-friendly later.” In that scenario an agile team would be hard pressed to create real value for users in a sprint. So what would be a better approach? Usability engineers or usability engineering know-how are necessary in an agile team. Usability engineers should work with requirements engineers and developers in a cross-functional, interdisciplinary (and non-hierarchal) project team. The bundling of different competencies creates a lot of potential for innovation. Different organizational forms such as delayed sprints which require an independent usability team to develop software concurrently are generally less efficient because they require synchronization and coordination effort.¹

Human performance capacity

A working knowledge of the human performance capacity is useful in the development of requirements and of a user interface that broadly meets the users’ needs in a system or product. In this regard it is helpful to get familiar with perception psychology. It suggests that the following principles generally underpin activities:

  • Acting (hammering a nail into a wall),
  • Perceiving (look at the nail and wall and feel them),
  • Making an evaluation (is the nail deep enough in the wall?),
  • Making a decision about future activities and
  • Acting again (nail is lodged deep enough in the wall. Mission accomplished. Nail still won’t hold, we continue hammering).
A metaphor for the foundational principles of perception

A metaphor for the foundational principles of perception

Evaluation and controlling become more precise the more our perception improves. In order for the controlling to be effective the necessary information must be available and presented in a form that matches the actor’s qualifications. When it comes to working with a dialog oriented system the information on the screen should have the following properties:

  • It must be presented in such a way that expectations match the users’ experiences
  • It must be presented in such a way that the users’ sense of recognizability and associability can be used
  • It must be encrypted in such a way that users can easily recognize symbols, icons, short-cuts etc. and can know what they mean,
  • It must be so comprehensive that the user has sufficient information for the task at hand – both for present goal of the production and for the future workflow,
  • It must be differentiated enough in terms of its content and representation – in accordance the user’s abilities.

Human performance capacity has its limits and these should be taken into consideration when designing a user-interface – whether it be with a mouse, keyboard or touch screen.²

  • Everybody makes mistakes, and they happen more often than one thinks – even among practiced users. Think about your own experience. Have you ever clicked on or “touched” something by mistake? What does that tell you? For one, you shouldn’t rely on your users’ practice level, instead try to reduce the number of possible mistakes and develop a well-thought out error correction mechanism.
  • Everything takes time – even perception. We require around a tenth of a second to perceive an object on the screen. A quarter of a second is required to divert our attention from one object to another – it takes about 1,25 seconds to make a choice between alternatives.
  • And that’s where the mouse, keyboard or touch screen come into play. Typing takes between 0,1 and 1,25 seconds per character. The time required for a mouse movement depends on the size of the object being selected but on average it takes about 1,5 seconds. And then there’s the switching between the keyboard and the mouse or between the keyboard and touch screen – generally between 0,36 seconds. All of these small elements add up and play a big role in a user’s productivity at a work station.
  • Magic number seven. The so-called 7+-2 rule has served us well in the area of structured methods. It was developed by George A. Miller in 1956 – it’s quite old, but it hasn’t lost any validity. The rule states: People can only retain around 7+/-2 items in their short-term memory. Try the rule on yourself: can you remember the names of seven people who you’ve just met? Are you able to recall more than seven possible errors at the same time when testing a program? The author revised his theory in 1975 and when he did so he started referring to three chunks. These chunks are comprised of three elements – but this revision does not generally change anything about our understanding of short-term memory. ³
  • Have you ever had the experience of bumping into someone? You recognize the person, that is to say, you know the face but you just can’t remember the name? We find it easier to recognize something than to recall even the smallest detail from our memory. What this means for the usability of an application is that identifying and selecting information in a list is much easier than entering information that needs to first be recalled by your memory.
  • Everything has to be learned. Every new task and tool needs to be learned first. It becomes more difficult when familiar terms and old habits have to be given up. So one should keep the technical terminology to a minimum and assign names to objects that make them easily identifiable.

Human perception

Beyond the knowledge about human performance capacities it is also useful to know how we perceive things. The discipline of form theory is devoted to these considerations. Max Wertheim, the founder of form theory, formulated a few laws of perception that are interesting for the development of requirements in a user interface.

The human brain automatically forms groups when perceiving figures such that all partial elements can be classed together. That’s where the law of good design comes in. It states that when we build visual groups we generally do so with reference to symmetry, simplicity, recurrence, coherence and balance. Our brain creates order. If the depiction allows for several forms of categorization then the interpretation that allows for the simplest, most unified design generally wins. Let’s test that out in an example.

Example of the law of good form

Example of the law of good form

The image on the right is perceived as being simpler. That is because of the coherence of the figures: we perceive two geometric figures.

The law of good design is complemented by the principles of inner unity and figure differentiation. The principles of the inner unity define criteria that help us categorize elements so that they are perceived as a group. The criteria are as follows:

  • Similarity: Similar or identical elements are perceived as a group.Similarity creates groups and improves our perception (figure A is irregular, figure B groups similar elements).
  • Proximity: In a set of similar elements, groupings will occur among objects that are close to each other.


    Proximity creates groups among similar items.

  • Coherence: Elements that form a figure that is coherent are perceived as a unit.
    Coherence example

    Coherence example

    Coherence creates a group.

  • Good improvements: elements that are ordered in simple, rule-based and constant succession are perceived as belonging to one another.
    Continuity example

    Continuity example

    You probably perceived two lines crossing each other, as opposed to two V form elements that meet at a single point.

The principle of figure differentiation states that perception is easier if a group of elements is recognized as foreground figures – all other elements are seen as background. You probably know the example below because it causes our perception to hover between the two states – it violates this principle.

Example of wavering perception

Example of wavering perception

Good figure differentiation is specifically useful for the development of icons and other important graphical elements. A figure is considered to be in the foreground if:

  • It’s a small element on a large surface
  • A dark element on a bright background
  • An element in the middle of the area, instead of the border,
  • An element with vertical or horizontal main axis
  • A symmetrical element, with a vertical axis of symmetry
  • An element curved towards the outside as opposed to curving inwards

Studies have shown that consistently applying these laws can reduce users’ reactions and decision making times by 30 percent. This foundational knowledge should equip you to develop good usability requirements. In a future blog post we will be looking at how you and your team can develop usability requirements in a collaborative context.



[1] More information can be found at Gothelf, J., & and Seiden, J. (2015). Lean UX – Mit Lean Methoden zu besserer User Experience. By p Publishers GmbH & Co. KG.
[2] More information can be found at Constantine, L. L., & Lookwood, L. A. (1999). Software for Use – A Practical Guide to the Models and Methods of User-Centered Design. Addison Wesley ACM Press.
[3] Miller, G. A. (No. 2. Vol. 63 1956). the Magical Number Seven, Plus or Minus Two: Some Limits on our Capacity for Processing Information. Psychological Review, S. pp. 81-97., and Miller, G. A. (1975). The Magic Number Seven after Fifteen Years. John Wiley & Sons, Inc.

What makes a successful Project Management process? How do I find the right business process? Which methods are useful? Ursula Meseberg is a graduate mathematician and co-founder of microTOOL. She is fascinated with current trends and has made a name for herself as an author of professional articles.

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