Interaction: User Capability-Product Demand Theory

The ideas of user capability and product demand provide a useful framework for design evaluation where the sensory, cognitive and motor demands made by a product are compared to the capability levels of the target user population (Bridger, 2003; Clarkson & Keates, 2003b). This theory of user-product compatibility is well known and forms the basis of the field of ergonomics and human factors (Bridger, 2003; Karwowski, 2002). These concepts are shown diagrammatically in Figure 4-1.

Figure 4-1 An illustration of the relationship between user sensory, cognitive and physical capabilities and the demands made on the user by the product

67

When a user interacts with a product, there is a cyclic process of perception, cognition and action through time in a given physical and social context, as shown in Figure 4-2 (Monk, 1998; Norman, 2002). The effects of previous actions are first perceived and interpreted according to expectations. These interpretations are then evaluated in terms of the user’s previous intentions and current goals. Based on this comparison, an intention is formed, and this intention is further operationalised into a sequence of actions that can satisfy the

intention. This mental plan is then executed on the product, which changes its state. At this point, the cycle repeats itself. In this way, a user moves through successive cycles from a starting state to an end state where hopefully a goal will be achieved.

Figure 4-2 The cycle of interaction: perception, cognition and action after (Norman, 2002)

The stages of perception rely on human sensory capabilities such as vision, hearing, taste, smell and touch. The stages of interpretation, evaluation, intention to act and sequencing of actions all rely on human cognitive capabilities such as working memory, long-term-memory and the processing capabilities of the human mind. Execution of the planned action sequence relies on human motor capabilities such as speech, motor control and strength in the limbs to physically perform actions in the world. Such frameworks appear in the literature embodying similar concepts with different levels of detail.

A particularly comprehensive framework for product interaction was developed by

Freudenthal (1999). It includes the directionality of changes in sensory, cognitive and motor capabilities with age (Figure 4-3). As evident from the diagram, the majority of capacities tend to decrease with increasing age. The components and scientific data to back up such frameworks are drawn from diverse fields including cognitive and social psychology, ageing

68

studies, disability studies, biomedical sciences and ergonomics research. As the theoretical knowledge base is built up from these constituent disciplines, frameworks for the architecture and function of the human element in the interaction system evolve to integrate new findings.

Therefore, frameworks such as the one presented in Figure 4-3 can be considered to be evolving meta-frameworks with interacting sub-systems.

Figure 4-3 A Framework for Senior Product Interaction (Freudenthal, 1999)

On the product side of the interaction equation, demand levels are set by the attributes of the product interface features. For example, a textual display on the product chassis will be designed with a certain text size, font style and colour, all placed on the product surface of another colour and material finish. This combination of design attributes sets a level of visual

69

demand on the user. In a similar way, combinations of other attributes lead to cognitive and motor demands. Therefore, using a capability-demand framework as presented in Figure 4-1 is a useful starting point in considering analytical evaluation methods for Inclusive Design.

By focusing on ways to measure product demands and relate them to measures of user capabilities, an estimate of the compatibility or fit between user and product could be established.

Even so, there is an important element to the interaction framework that requires explicit attention. User capabilities and product demands are always linked by the task that is to be performed. Carroll (1993) defines capability as follows: “As used to describe an attribute of individuals, ability refers to the possible variations over individuals in the liminal levels of task difficulty (or in derived measurements based on such liminal levels) at which, on any given occasion in which conditions appear favourable, individuals perform successfully on a defined class of tasks”. Carroll further defines a task as: “… any activity in which a person engages, given an appropriate setting, in order to achieve a specifiable class of objectives, final results, or terminal states of affairs.” Thus a cognitive task and ability is defined as: “…

any task in which correct or appropriate processing of mental information is critical to successful performance. A cognitive ability is any ability that concerns some class of

cognitive tasks, so defined.” Following this definition, a sensory and motor capability can be defined similarly as an ability that concerns sensory and motor tasks (Carroll, 1993;

Fleishman & Quaintance, 1984; Kondraske, 2006a).

In other words, from these definitions, capability could only be understood when taking into account the task being performed and the level of performance required for success in that task. The level of performance required is set by the parameters of the task, which, in keeping with the capability-demand model, will be the product demand. The main point is that the system of user, product, task and environment and their interactions must all be considered.

Capability variation is the variation in the threshold levels of performance among different people in the population of interest. In addition, capabilities and tasks are hierarchical in nature i.e. capabilities and tasks could be broken down into sub-capabilities and sub-tasks.

Therefore, in operationalising user capabilities for measurement, a level of granularity must be decided. This issue is explored further in the next section on modelling human functional capability.

70