Human-Computer Interaction 3e ­ Dix, Finlay, Abowd, Beale

exercises  -  1. the human


Devise experiments to test the properties of (i) short-term memory (ii) long term memory, using the experiments described in this chapter to help you. Try out your experiments on your friends. Are your results consistent with the properties described in this chapter?


The aim of this exercise is to get the student to think about experimental design. The experiments devised can effectively be repeats of the originals. Chapter 9 can be used for reference on experimental design.

(i) Short term memory (STM)

The student should first choose an aspect to investigate: for example, digit span, recency effect, decay.

Example solution: STM decay

ideally selected to represent population, more probably undergraduate students (try to get a range of academic subjects).
Sample size: 10+
  • split subjects into two groups. Each subject studies list of 15-20 words (could try with both nonsense words and actual words to see any difference). Subject has to recall list either (a) immediately or (b) after 20 second delay. Measure the number (or percentage) of the words remembered correctly.
  • A within-groups design can be used to avoid individual bias or group variation (as long as different lists are used for each attempt).
  • independent variable -- delay in recall
    dependent variable -- number correctly recalled.
  • Group (b) should be given a task to do during the delay period in order to avoid rehearsal. If possible this task should occupy a different channel to minimise interference, e.g. a visual recognition task.
Those in (b) will perform worse than those in (a) since STM will decay.
graphs to see decay.
T test

(ii) Long term memory (LTM)

The student should first choose an aspect to investigate: for example, the total time hypothesis or the distribution of practice effect.

Example solution: distribution of practice effect

as above. Should have no prior experience of the skill to be learned.
  • split subjects into 3 groups. Each subject must learn a skill (for example shorthand or typing -- must be measurable). Group A learns for 1 hour a week for 6 weeks. Group B learns for 2 hours a week for 3 weeks. Group C learns for 3 hours a week for 2 weeks.
  • After each group's training is complete the subjects are tested and the number of mistakes made are noted.
  • Between-groups design.
  • independent variable - style of learning
    dependent variable - accuracy
Group A will be best (due to the distribution of practice effect)

(N.B. This one is not easy to run but could be done with cooperation from friends)



Observe skilled and novice operators in a familiar domain: for example, touch and 'hunt-and-peck' typists, expert and novice game players, or expert and novice users of a computer application. What differences can you discern between their behaviours?




From what you have learned about cognitive psychology, devise appropriate guidelines for use by interface designers. You may find it helpful to group these under key headings: for example, visual perception, memory, problem solving, etc, although some may overlap such groupings.


Guidelines are just what they say they are: guidelines. They provide for a consistent look and feel for an interface, as well as trying to exclude the more obvious mistakes that can be made from a psychological perspective. However, there are occasions when such constraints should be broken; for new interaction devices, for example, or to create a unique style of product.

Because of this, there is no one correct answer to this question: some will be more cognitively friendly than others, that is all. Guidelines can range from the general principle type shown below down to highly detailed information on what each component in a display should look and behave like.

Some examples of guidelines with cognitively solid foundations are shown below - this is not an exhaustive set by any means

This exercise should encourage students to look into the literature on human factors, cognitive psychology and human physiology, and come up with some hard evidence about human limitations. This can then be used to provide informed guidelines.



What are mental models, and why are they important in interface design?

answer available for tutors only

Mental models are the theories people build to understand the causal behaviour of systems. These are often partial, unstable and subject to change. They may be internally inconsistent. They may be superstitious and based on incorrect interpretation of evidence. They are important, as errors can occur if the user's model is incorrect or if the designer has a different model than the user. One way of minimising problems is supporting conventions - another is to make the correct model explicit.



What can a system designer do to minimise the memory load of the user?

answer available for tutors only

Discuss two ways of remembering - recognition is the knowledge that you have seen something presented to you; recall is the reproduction of something from memory. Recognition is simpler - the information required is provided as the cue. Recall is more complex as the information has to be retrieved from memory but cues can help this - e.g. using categories or images. The interface designer can where possible allow recognition by providing information up front (e.g. labelled buttons). Where this is not possible support recall by using cues such as iconic images, categories of menu item. The answer may also discuss short term memory where chunking and restricting number of items are important.



Human short-term memory has a limited span. This is a series of experiments to determine what that span is. (You will need some other people to take part in these experiments with you - they do not need to be studying the course - try it with a group of friends.)

(a) Kim's Game

Divide into groups. Each group gathers together an assortment of objects - pens, pencils, paper-clips, books, sticky notes, etc. The stranger the object, the better! You need a large number of them - at least 12 to 15. Place them in some compact arrangement on a table, so that all items are visible. Then, swap with another group for 30 seconds only and look at their pile. Return to your table, and on your own try to write down all the items in their pile.

Compare your list with what they actually have in their pile. Compare the number of things you remembered with how the rest of your group did. Now think introspectively: what helped you remember certain things? Did you recognise things in their pile that you had in yours? Did that help? Do not pack the things away just yet.

Calculate the average score for your group. Compare that with the averages from the other group(s).

Questions: What conclusions can you draw from this experiment? What does this indicate about the capacity of short-term memory? What does it indicate that helps improve the capacity of short-term memory?

(b) "I went to market..."

In your group, one person starts off with "I went to market and I bought a fish" (or some other produce, or whatever!). The next person continues "I went to market and I bought a fish and I bought a bread roll as well". The process continues, with each person adding some item to the list each time. Keep going around the group until you cannot remember the list accurately. Make a note of the first time someone gets it wrong, and then record the number of items that you can successfully remember. Some of you will find it hard to remember more than a few, others will fare much better. Do this a few more times with different lists, and then calculate your average score, and your group's average score.

Questions: What does this tell you about short-term memory? What do you do that helps you remember? What do you estimate is the typical capacity of human short-term memory? Is this a good test for short-term memory?

(c) Improving your memory

Try experiment 1.6(a). again, using the techniques on page 39.

Has your recall ability improved? Has your group's average improved? What does this show you about memory?


open-ended experiments



Locate one source (through the library or the Web) that reports on empirical evidence on human limitations. Provide a full reference to the source. In one paragraph, summarize what the result of the research states in terms of a physical human limitation.

In a separate paragraph, write your thoughts on how you think this evidence on human capabilities impacts interactive system design.


open-ended research


EXERCISE 1.8 [extra - not in book]

What is the difference between recognition and recall in relation to human memory? Discuss the implications of this for interface designers.

answer available for tutors only

Recognition is the knowledge that you have seen something presented to you. Recall is the reproduction of something from memory. Recognition is simpler - the information required is provided as the cue. Recall is more complex as the information has to be retrieved from memory but cues can help this - e.g. using categories or images. Implications for interface designers include the need, where possible, to allow recognition by providing information up front (e.g. labelled buttons). Where this is not possible support recall by using cues such as iconic images, categories of menu item.


EXERCISE 1.9 [extra - not in book]

"A little psychology is worse than none at all". Do you agree with this statement? Justify your stand in the context of designing usable interactive systems.

answer available for tutors only

There are different ways of answering this question depending on the stand you take. One problem with knowing "a little" psychology is the danger of applying psychological principles out of context and making simplistic judgements. Principles and guidelines that derive from psychology are context dependent. For example, predictability is a principle that might not apply to a computer game. So it is important that the underlying theory is interpreted appropriately and that the context in which the rule is applied is comparable to that of the theory. An example of misuse of psychology is in the application of the 7+/- 2 short term memory limit to menu design. Menus support recognition by presenting the options to the user - this is not a task that relies on short term memory. Therefore the oft-heard recommendation to limit menu items to 7+/- 2 is a misapplication of psychology. Try to find more examples of well and badly applied psychology for yourself.


EXERCISE 1.10 [extra - not in book]

Is it important that system designer takes account of individual differences when creating an interactive system? Justify your answer.

answer available for tutors only

Yes - although there are generalisations that can be made about human capabilities and limitations, individuals also differ on long and short term factors and some that change over time. Long term factors include gender, learning style, intellect etc. Short term factors include stress, fatigue. Factors that change over time include age, which of course effects physical capabilities, possibly intellectual (eg memory). Ignoring these for a user group will cause problems.


EXERCISE 1.11 [extra - not in book]

What is the difference between a slip and a conceptual error? How might a designer minimise the occurrence of both among users of a system?

answer available for tutors only

A slip is an error that occurs when the context of skilled behaviour is changed. A conceptual error occurs due to lack of understanding on the part of the user. This may be a result of an inadequate or incorrect mental model. Answer should discuss ways of supporting mental models - e.g. transferring user's knowledge from other domains, using familiar objects, maintaining consistency between platforms, providing feedback, etc. Slips can be avoided by ensuring consistency, taking account of stress in skilled operation, etc.


EXERCISE 1.12 [extra - not in book]

How might you use the notion of reward in interface design to increase the positive emotional response of users? Can you find any examples of this?

answer available for tutors only

Reward is essentially positive reinforcement of desired or good behaviour. This could be through providing explicit praise (used frequently in educational systems when a correct answer is given) or through more implicit elements that engage or entertain the user. Novelty, social interaction, feedback and surprise are all potentially rewarding to the user. For example, seeing a direct relationship between action and effect (feedback) can be rewarding. However care needs to be taken because each of these things can backfire and become irritating if not used properly. An example of this is the Microsoft Office paperclip (see Chapter 11), which attempts to motivate users by making getting help a novel social interaction, was actually irritating to users and was eventually dropped.


EXERCISE 1.13 [extra - not in book]

Describe Fitts’ law. Describe which implications Fitts’ law has on user interface design. 

answer available for tutors only


EXERCISE 1.14 [extra - not in book]

Consider the train booth scenario. Give four examples of knowledge stored in LT memory, e.g. declarative, procedural, semantic and episodic (4 points).

answer available for tutors only

Declarative knowledge – current station, destination station

Procedural knowledge – how to interact with a system through a card (magnetic or chip based)

Semantic knowledge – any idea of the itinerary

Episodic knowledge – the first trip by train, or any memories related to the destination location.

Individual exercises

ex.1.1 (ans), ex.1.2 (ans), ex.1.3 (ans), ex.1.4 (tut), ex.1.5 (tut), ex.1.6 (open), ex.1.7 (open), ex.1.8 (tut), ex.1.9 (tut), ex.1.10 (tut), ex.1.11 (tut), ex.1.12 (tut), ex.1.13 (tut), ex.1.14 (tut)

Worked exercises in book


Suggest ideas for an interface which uses the properties of sound effectively. [page 24]


Identify the goals and operators involved in the problem 'delete the second paragraph of the document' on a word processor. Now use a word processor to delete a paragraph and note your actions, goals and subgoals. How well did they match your earlier description? [page 45]


Produce a semantic network of the main information in this chapter. [page 54]

  • a worked exercise

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