Monday, August 16, 2010

Affordances 1

Affordances and physicality

Objects physical form determines peoples’ expectancy about possible actions to be carried out with them. “The physical form of an interface fundamentally shapes the kinds of interactions that users can and will perform” Benford et al, 2005, p. 7. This idea grasps on the Gibsonian notion of affordances (Gibson, 1977, 1979). They (affordances) are “properties of the world that make possible some actions to an organism equipped to act in a certain way” (Gaver, 1991, p. 80). Norman (1999) described two different kinds of affordances: physical or real affordances and perceived affordances. They may but do not necessarily overlap. Physical affordances are built in the computational artifact subcomponents (keyboard, display screen, pointing device, and selection buttons (e.g., mouse buttons) that affords pointing, touching, looking, and clicking (Norman, 1999). They are independent of the perceiver; they are relationships between the real world and the perceiver. Perceptual affordances refer to that class of affordances for which there is perceptual information for an existing [real] affordance (Gaver, 1991). They are things that are visible on the screen (i.e. an icon); they are “visual feedback that advertise the affordances” (Norman, 1999, p. 40). Perceptual affordances play an important role in the world of screen-based products as well as cultural conventions. The latter are socially established practices. The changing form of a cursor, specifying what actions are possible at a given moment with a given icon, is an example of a cultural convention for interface design. Conventions work only if they are known to the user, if they have been learned, unlike the affordances that do not involve memory or inference (Norman, 1999).

When “putting” “affordances” in the world (interface), people can make mistakes. Gaver (1991) wrote, “Distinguishing affordances from perceptual information about them is useful in understanding ease of use” (p. 80). He showed that in respect to the existence of an affordance and the existence of perceptual information for it there are three classes of affordances: (1) perceptible affordances (perceptual information is available for an existing affordance), (2) hidden affordances (there is no information available for an existing affordance, and must be inferred from other evidence), and (3) false affordances (information suggests a nonexistent affordance). The fourth class is that of correct rejections. People will usually not think of a given action when there are no affordances for it or any perceptual information suggesting it (p. 80). They may think of it if they desire a certain action (see below the differentiation between expected, sensed, and desired movements).

Figure 1. Separating affordances from the information available about them (Gaver, 1991, p. 80)

The notion of affordance is very promising for interface design, because it implies that if the affordance exists and is perceived, then a person acting on that interface will know how to use it without relying on memory (retrieve cultural conventions) or thinking (inferring information). Real affordances are in the world; they rely on the physical shape of the objects. Therefore, it is important to bring physicality back by designing devices based on perceived physical affordances rather than on cultural conventions.

Physicality is a new trend in User Interfaces considered by Norman (2007) to be the an important UI breakthrough. The main idea of physicality is the use of physi­cal controls and devices, where “we control things by physical body movement, by turning, moving, and manipulating appro­priate mechanical devices” (Norman, 2007, p. 43), i.e. tuning the radio by turning a knob instead of acting over a complex graphical interface. It is not a reversion to mechanical controls; it is a shift to physical devices that are “coupled with intelligent, embedded processors and communication” (Norman, 2007, p. 47).


Gaver, W. (1991). Technologie affordances. Paper presented at the Conference on Human Factors in Computing Systems, New Orleans, Louisiana, Unated States.

Gibson, J. J. (1977). The theory of affordances. In R. Shaw & J. Bransford (Eds.), Perceiving, Acting, and Knowing: Toward an Ecological Psychology (pp. 62-82): Hillsdale, NJ: Erlbaum.

Gibson, J. J. (1979). The Ecological Approach to Visual Perception. Boston: Houghton Mifflin.

Norman, D. A. (1999). Affordances, conventions and design. interactions, 6(3), 38-43.

Norman, D. A. (2007). The Next UI Breakthrough, Part 2: Physicality. interactions, 14(4), 46-47.