Affordance Theory
Affordance theory was originally developed by James Gibson, a psychologist interested in perception. Affordances were originally defined as ‘action possibilities’ between an animal and its environment. Specifically, the term affordance (clues in the environment) was used to indicate an action possibility that was sensed in an immediate, direct way with no sensory processing required. As an example of this construct, a slide control or push button would, it is claimed, be directly understandable and require no sensory processing. Affordances always exist as a relationship between an organism and its environment. Whilst looking to scramble up a steep, grassy slope trees afford grip to haul you up, rocks afford grip to propel. They also have to be usable, affordances do not exist if they cannot by physically used through lack of height for example. This notion of direct, immediate access to the ‘meaning’ of an affordance without sensory processing is obviously appealing to designers of products. It was popularised in human computer interface circles after Donald Norman used the concepts in Psychology of Everyday Things.
In Norman’s view of interface design the notion of affordance was used alongside conceptual models and conventions to aid a designer. However, as interest in affordances grew he became concerned that discussion about them in hci circles was wandering further and further away from his original intention. Norman has expressed his dissatisfaction on this and distinguishes conceptual models, real affordances, perceived affordances, constraints and conventions.
Conceptual models provide the logic for how an interface works and provide a base for reasoning about possible actions in an interface. Real affordances are all the affordances that physically exist, but may not actually provide access to a designer’s intention. Perceived affordances are those that the designer has managed to make readily accessible and understandable to the user of the interface. Constraints exist in physical and logical form – an example of a physical constraint would be where a section of a monitor does not provide cursor feedback so it’s clear that no actions are possible in that areas. A logical constraint allows reasoning to be made about possibilities for example where a user is asked to click on five locations, but only four are visible. The user knows logically that another location must exist and can look for it using, e.g., scroll bars. Scroll bars are in turn examples of cultural conventions which have become to be accepted within communities. They are understood precisely because of their ubiquitous nature which has developed over time.
Norman is very clear on his wish to see these different aspects of interface design clearly separated out to help analysis and subsequent design. It is clear how an individual designer’s role can be much stronger in the development of intended perceived affordances of a product (they can have direct influence on this) whilst it is more difficult to change cultural constraints (at least in the short term).
