Models provide us with a representation which is easier to use and manipulate than the thing we are designing (that the model represents). This means that it is easy to test out different designs to see which is best for a given purpose. We can also use models to model a situation e.g. the social situation in which the thing we are designing will be placed. This helps us to understand how the new system will impact on the current working situation. Finally, we can use models to provide approximations of human behaviour which can be used to determine whether our designs will be difficult to use, without involving users.

Models contribute in two main ways – generative or evaluative. Evaluative allow us to assess whether the final product will meet some criteria. Generative models provide input to design – it should be noted that most models have some generative aspect to them.

Use centred requirements analysis employs models to represent a typical user’s work environment. This ranges from representations of the stakeholders and their perspectives to considerations of how systems influence the social and organisation situation. Cognitive models, on the other hand, provide a simplistic view of how user might react to situation, and in turn, how they might use a newly designed system.

Both socio-technic and SSM model the stakeholders involved in the development of a new system. These models give designers a grasp on the context of who are involved in the development of the new system, and what impact this will have on their working environment. The primary difference between socio-technic and SSM approaches is that SSM takes a wider view of the context. This means that it provides a model of not just the stakeholders, but also the environment in which they work. Providing such a wider view allows designers to get a grasp on the wider picture, but does involve extra effort on the designers’ and analysts’ part to develop and understand the models.

The crucial difference between participatory design and socio-technical and SSM approaches is that participatory design is a design philosophy in which users are considered central and are included on the design team throughout the stages of development. Socio-technical and SSM approaches are concerned with establishing models of user requirements (as opposed to necessarily including them in the design process). In contrast, models used in participatory design are used to support communication between designers and users e.g. storyboards which model typical user interactions.

GOMS uses a Model Human Processor to model human cognitive processes. This is used in conjunction with models describing Goals, Operators, Methods, and Selection rules to determine execution times using a Keystroke Level Model. This provides predictions of task execution time for tasks that involve no user errors or problem solving.

GOMS’ model of human cognition is referred to as the Model Human Processor. Both models assume that there are interrelated information processors, but ICS’ model is much more complex. Essentially the representational domain of ICS (containing 4 subsystems) is the cognitive processor of GOMS.

GOMS attempts to make quantitative predictions of task execution time by expert users. This takes no account of errors, learning, or problem solving. ICS, on the other hand, can model these issues, but does not give simple quantitative predictions. As such ICS is intended to be used by expert psychologist evaluators, whereas GOMS was intended to be used by trained evaluators.

The following is a definition of the stakeholders concerned with the introduction of a new cash machine:

A storyboard for this activity is as below:

Goal – withdraw cash.

  Method - Enter PIN

    Operators – press-1st digit of PIN, press-2nd digit of PIN, press-3rd digit of PIN, press-4th digit of PIN

  Method - Select withdrawal

    Operator – select ‘withdraw cash’

  Method - Select amount

    IF <amount is shown> THEN <Operator - select amount>

    ELSE

      <Operators - select ‘other amount’, press-1st digit of amount, press-2nd digit of amount>

    IF <3 digit amount of cash> THEN <Operator – press-3rd digit of amount>

  Method - Collect card & money

    Operators – take cash, take card

One approach would be to explicitly model each subsystem of ICS and connect them via data pathways. Analysts could then feed in different kinds of information to the sensors, indicated which subsystems they think might be used, and then observe how the data flows around the network. This would reduce the burden of working out how the data flows around the system.

A more comprehensive approach would be to use an expert system (a system which contains a lot of knowledge about an area, and some artificial intelligence) to help work out which subsystems might be involved in processing information. This helps reduce the problems of deciding which subsystems and representations are likely to be used.