An affordance is the design aspect of an object
that suggests how the object should be used (McGrenere & Ho,
2000). For
example, when we hold a pencil in such a way that it fits comfortably in the
hand, the pencil affords being held in this way as a result of its length,
width, weight and texture, all with respect to the size, configuration and
musculature of the hand (Allard, 2001).
Affordances
provide strong clues to the operation of things. Knobs are for turning, balls are for throwing or bouncing and
chairs are for sitting on. When
affordances are utilized, the user knows what to do by just looking; no
instructions or pictures are required (Allard,
2001). Affordances are made in everyday life, including the computer
world. For example, the surface of the computer screen is not only important in
the interpretation of the user’s action, it is also important in the user’s
understanding of the interface behaviours (Gallant
et al, 2001).
The word ‘affordance’ was originally invented by the perceptual
psychologist J.J. Gibson (1977, 1979 cited from Allard,
2001). Gibson made it his life’s work to describe an appropriate ecological
frame of reference (McGrenere & Ho, 2000). He believed that studying the animal’s visual perception in isolation
from the environment that is perceived results in false understandings. He claimed that we perceive at the level of mediums, surfaces and
substances rather than at the level of particles and atoms and, in particular,
we tend to perceive what the combination of mediums, surfaces and substances
offer us (McGrenere & Ho, 2000). Thus,
as defined by Gibson (1979 cited from Allard,
2001), ‘the affordance of the environment are what it offers
the animal, what it provides or furnishes, either good or ill’.
There are three fundamental properties of an affordance described by Gibson. An affordance exists relative to the action capabilities of a particular actor, the existence of an affordance is independent of the actor’s ability to perceive it and an affordance does not change as the needs and goals of the actor change (McGrenere & Ho, 2000). Gibson had focussed his work on direct perception; a form of perception that does not require mediation or internal processing by an actor. He indicated that direct perception was possible when there was an affordance and there was information in the environment that uniquely specified that affordance (McGrenere & Ho, 2000).
Norman (1988) described affordance as referring ‘to the perceived and actual properties of the thing, primarily those fundamental properties that determine just how the thing could possibly be used’. Norman talked of both perceived and actual properties and implied that a perceived property may or may not be an actual property, but it is still an affordance. He further indicated that an affordance refers primarily to the fundamental properties of an object (McGrenere & Ho, 2000).
Further clarification on Norman’s position, he stated that affordances ‘provide strong clues to the operations of things’ and that they ‘suggest the range of possibilities’ (Norman, 1988). He argued that when designers take advantage of affordances, the user knows what to do just by looking. Although complex things may require supporting information, simple things should not (Norman, 1988). If they do, the design is considered to be a failure.
Norman (1988) work was primarily concerned with perceived affordances; what the user understands the affordances to be. Perceived affordance is a combination of what you see, and what you know (your conceptual model). A designer creates visible structures that convey the conceptual model and its affordances (Norman, 1988). The designer cares more about what actions that user perceives to be possible than what is true (Chuong et al, 2001).
In product design, there can be both real and perceived affordances. As an example, in graphical, screen based interfaces, the designer primarily can only control perceived affordances. The computer system already comes with built-in physical affordances (Chuong et al, 2001).
Much of the discussion about the use of affordances is addressing conventions, or cultural constraints. Three behavioral constraints exist: physical, logical and cultural. A description and example using graphical screen layout is depicted.
Physical constraints are closely related to real affordances. For example, it is not possible to move the cursor outside a screen; this is a physical constraint. Logical constraints use reasoning to determine the alternatives. If we ask the user to click on five locations and only four are immediately visible, the user knows, logically, that there is one location off the screen. Logical constraints are valuable in guiding behaviour.
Cultural constraints are conventions shared by a cultural group. The fact that the graphic on the right hand side of a display is a ‘scroll bar’ and that one should move the cursor to it, hold down a mouse button, and ‘drag’ it downward to see objects located below the current visible set is a cultural, learned convention. A convention is a cultural constraint, one that has evolved over time and require a community of practice. They are slow to be adopted, and once adopted, slow to go away.