2. the computer


Individually or in a group find as many different examples as you can of physical controls and displays.
(a) List them
(b) Try to group them, or classify them.
(c) Discuss whether you believe the control or display is suitable for its purpose. (section 3.9.3 may also help)


open-ended investigation



Exercises 2.2 and 2.3 involve you examining a range of input and output devices in order to understand how they influence interaction.

A typical computer system is comprised of a QWERTY keyboard, a mouse, and a colour screen. There is usually some form of loudspeaker as well. You should know how the keyboard, mouse and screen work - if not, read up on it.

What sort of input does the keyboard support? What sort of input does the mouse support? Are these adequate for all possible applications? If not, to which areas are they most suited? Do these areas map well onto the typical requirements for users of computer systems?

If you were designing a keyboard for a modern computer, and you wanted to produce a faster, easier to use layout, what information would you need to know and how would that influence the design?

answer available for tutors only

The keyboard supports a single type of event, a keypress, which includes data that signals the value of the key pressed. This data is represented as an ASCII key value or some other international standard representation, such as UNICODE. Some keys on a keyboard introduce new modes, or interpretations of keystrokes, so you can use the same key to send uppercase and lowercase letters, or introduce special commands or modifiers (such as Control-A). Many keyboards also include special function keys, which produce non-ASCII keyboard events that must be handled in special ways. A mouse provides two main pieces of information. One piece of information is the location, in screen coordinates, of the mouse pointer. The other piece of information is any event information from the pressing and releasing of buttons of the mouse. Certain mouse designs include additional inputs, such as a wheel that sends discrete directional scroll events.

The information needed to redesign keyboard layout would include the frequency of letters or commands to be issued by the keyboard as well as empirical data on motor actions of the hands and fingers in performing typing actions. Various modified keyboard layouts do exist, such as the DVORAK keyboard, but none has been successful in supplanting the QWERTY standard.



Pick a couple of computer input devices that you are aware of (joystick, light pen, touchscreen, trackball, eyegaze, dataglove, etc.) and note down how each has different attributes that support certain forms of interaction. You ought to know a little about all of these devices - if you do not, research them.


open-ended investigation



What is the myth of the infinitely fast machine?

answer available for tutors only

The adverse effects of slow processing are made worse because the designers labour under the myth of the infinitely fast machine http://www.hcibook.com/alan/papers/hci87/. That is, they design and document their systems as if response will be immediate. Designers should plan explicitly for slow responses where these are possible. A good example, where buffering is clear and audible (if not visible) to the user, is telephones. Even if the user gets ahead of the telephone when entering a number, the tones can be heard as they are sent over the line. This type of serendipitous feedback should be emulated in other areas.



Pick one of the following scenarios, and choose a suitable combination of input and output devices to best support the intended interaction. It may help to identify typical users or classes of user, and identify how the devices chosen support these people in their tasks. Explain the major problems that the input and output devices solve.

Environmental database

A computer database is under development that will hold environmental information. This ranges from meteorological measurements through fish catches to descriptions of pollution, and will include topographical details and sketches and photographs. The data has to be accessed only by experts, but they want to be able to describe and retrieve any piece of data within a few seconds.

Word processor for blind people

A word processor for blind users is needed, which can also be operated by sighted people. It has to support the standard set of word-processing tasks.

answer available for tutors only

The environmental database will be operated by skilled experts. It is likely that they will want geographic displays of information, so leveraging off of operations on a map seem likely to be intuitive. Various parameters would likely be overlaid on the map, so some intuitive way to select among parameters, possibly with a purpose-built keyboard with function keys representing the parameters to be revealed. There should be an analysis of current practices by these experts in which the way they like to view information is revealed and the way they like to manipulate information. It seems that an interface to encourage exploration to reveal trends in data would be useful, so how this exploration can be made natural would impact design recommendations for input devices.

The word processor for blind people that can also be used by sighted people is more challenging because you need to accommodate two very different modes of interaction. Blind users cannot rely on the visual domain. One approach is to take an existing word processor and attempt to modify it for non-sighted use. It is clear that some level of audio feedback would be useful. Perhaps a screen reader to assist in reading the contents of the text window. The prosody of the voice might indicate formatting of the text (for bold, italics, headings). Providing a chorded keyboard for input might be easier for the blind user, as orientation on a traditional keyboard might prove difficult without a lot of training. Two handed input techniques would also be useful, whereby one hand could be used for chording/typing text and the other for performing tasks. Audio feedback on commands would be useful, but the granularity of feedback would have to be experimented with.



Describe Fitts' Law (see chapter 1). How does Fitts' Law change for different physical selection devices, such as a 3-button mouse, a touchpad, or a pen/stylus? (You'll need to do some research for this.)

answer available for tutors only

There should be an attempt to write down Fitts' Law in one of its many forms. The form students may be most familiar with is:

Movement time = a + b log2(distance/size + 1)

The relationship between time and distance and size of a target should be expressed. Students should know what the time stands for (time for selecting a target) and that distance and size are descriptions of the target.

They should be able to adequately explain that the constants in the formula are there to calibrate for different selection techniques (3-button mouse, trackpad, pen, etc.). They may simply mention that the law changes for different selection mechanisms, or, more explicitly, mention a and b parameters.

NB There are more forms of Fitts' Law than the one above, but the important parts are the constants and the time/distance/size relationships.


EXERCISE 2.7 [extra - not in book]

Computer systems are constantly developing, and manufacturers keep encouraging us to buy the latest, fastest systems, with the newest designs of keyboards, screens, more memory and faster processors, and so on. Give examples of circumstances in which having the latest technology is advantageous, and give examples when it can cause difficulties.


When raw computing power is at a premium, having the fastest computing power available is ideal - this is true for scientific applications, numerical modelling and analysis, and so on. Such increases in power can turn what were once batch jobs into interactive ones, allowing the user to explore and develop 'what if' scenarios that can lead to a better understanding of the underlying problems.

Developers of software usually benefit from having a faster design-code-compile-debug cycle, allowing them to produce their code more rapidly. However, this can cause problems too in that they become used to the capabilities of their machines and expect that all users will have similar resources. This can lead to large programs that perform ponderously on the more conventional machines that ordinary users have to work on.


EXERCISE 2.8 [extra - not in book]

Identify input and output devices that could benefit users with special needs.


  • Speech input - suitable for physically handicapped people
  • Speech output - suitable for blind people or those with other vision problems
  • LCD screens - suitable for users with sub-optimal vision because they are clearer
  • Touchscreens - for users with lower than usual hand-eye coordination
  • Virtual reality input and output - for immersive treatment of phobias
  • Braille printer - hardcopy output for the blind
  • ... continue ...


EXERCISE 2.9 [extra - not in book]

Consider a typical modern video games console (such as a PlayStation or Xbox). This represents a computer in a highly interactive environment, and is one with a different type of input device from the standard mouse and keyboard. Draw a sketch of a particular console, labeling all forms of input. Explain why the choice of inputs might be appropriate for supporting video game interactions.

Explain how the different nature of the device makes it easier and more intuitive than the keyboard and mouse for all levels of user to interact with the system, because the device is tailored precisely to meet the needs of the user and the requirements of the system. It is interesting and instructive to analyse the input devices and output styles of many computer and arcade games, since they are often the result of careful and sensible design.

answer available for tutors only

open-ended investigation

Individual exercises

ex.2.1 (open), ex.2.2 (tut), ex.2.3 (open), ex.2.4 (tut), ex.2.5 (tut), ex.2.6 (tut), ex.2.7 (ans), ex.2.8 (ans), ex.2.9 (open)

Worked exercises in book


What input and output devices would you use for the following systems? For each, compare and contrast alternatives, and if appropriate indicate why the conventional keyboard, mouse and CRT screen may be less suitable. [page 105]
(a) portable word processor
(b) tourist information system
(c) tractor-mounted crop-spraying controller
(d) air traffic control system
(e) worldwide personal communications system
(f) digital cartographic system


What is the basic architecture of a computer system? [page 114]


How do you think new, fast, high-density memory devices and quick processors have influenced recent developments in HCI? Do they make systems any easier to use? Do they expand the range of applications of computer systems? [page 119]