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42 Chapter 2 Understanding and conceptualizing interaction

1. Instructing
This kind of conceptual model describes how users carry out their tasks through in-
structing the system what to do. Examples include giving instructions to a system to
perform operations like tell the time, print a file, and remind the user of an ap-
pointment. A diverse r.?nge of devices has been designed based on this model, in-
cluding VCRs, hi-fi systems, alarm clocks, and computers. The way in which the
user issues instructions can vary from pressing buttons to typing in strings of char-
acters. Many activities are readily supported by giving instructions.
Operating systems like Unix and DOS have been specifically designed as com-
mand-based systems, to which the user issues instructions at the prompt as a com-
mand or set of commands. In Windows and other GUI-based systems, control keys
or the selection of menu options via a mouse are used. Well-known applications that
are command-based include word processing, email, and CAD. Typically, a wide
range of functions is provided from which users choose when they want to do some-
thing to the object they are working on. For example, a user writing a report using a
word processor will want to format the document, count the numbers of words typed,
and check the spelling. The user will need to instruct the system to do these opera-
tions by issuing apprbpriate commands. Typically, commands are carried out in a se-
quence, with the system responding appropriately (or not) as instructed.
One of the main benefits of an instruction-based conceptual model is that it
supports quick and efficient interaction. It is particularly suited to repetitive kinds
of actions performed on multiple objects. Examples include the repetitive actions
of saving, deleting, and organizing email messages or files.

There are many different kinds of vending machines in the world. Each offers a range of
goods, requiring the user initially to part with some money. Figure 2.3 shows photos of two
different vending machines, one that provides soft drinks and the other a range of snacks.
Both support the interaction style of issuing instructions. However, the way they do it is
quite different.
What instructions must be issued to obtain a can of soft drink from the first machine and
a bar of chocolate from the second? Why has it been necessary to design a more complex
mode of interaction for the second vending machine? What problems can arise with this
mode of interaction?

Comment The first vending machine has been designed on a very simple instruction-based conceptual
model. There are a small number of drinks to choose from and each is represented by a large
button displaying the label of each drink. The user simply has to press one button and
(hopefully) this will have the effect of returning the selected drink. The second machine is
more complex, offering a wider range of snacks. The trade-off for providing more choices,
however, is that the user can no longer instruct the machine by using a simple one-press ac-
tion but is required to use a more complex process, involving: (i) reading off the code (e.g.,
C12) under the item chosen, then (ii) keying this into the number pad adjacent to the dis-
played items, and (iii) checking the price of the selected option and ensuring that the
amount of money inserted is the same or more (depending on whether or not the machine
provides change). Problems that can arise from this mode of interaction are the customer