The idea of affordance is really pretty simple. We use the term in everyday English, such as when we say, "That window affords us a beautiful view of the mountains." The technical way that we use the term in HCI and usability and in the UAF is essentially the same, except that we have given it some fine tuning and defined a few different kinds of affordances.
We define and use the terms cognitive affordance, physical affordance, and perceptual affordance to refer to parallel and equally important usability concepts for interaction design, and we argue for connecting these different kinds of affordance in any design context.
The relevant part of what my dictionary says about 'to afford' is that it means to yield, to give, or to furnish. For example, the study window in my house affords a fine view. By putting the user and purpose of the affordance into the picture, we get a more interaction- and user-oriented view: an affordance helps or aids the user in doing something. The study window in my house helps or aids me to enjoy a fine view, but I have to participate (I have to look through the window) to accrue the benefit of seeing that view.
Norman's own stages-of-action model (Norman 1986) describes the typical course
of interaction between a human user and a computer or any kind of machine. During
interaction, a user performs both physical and cognitive actions and requires
affordances to help with each. Norman calls these, respectively, real and perceived
affordances. In our work on the User Action Framework (Hartson, Andre, Williges,
and van Rens 1999, Andre, Belz, McCreary, and Hartson 2000, Andre, Hartson,
Belz, and McCreary 2001), based on Norman's model, we have also found a need
for both kinds of affordances in the context of interaction design and usability.
Since affordances are features that help users perform actions, we named the
kinds of affordance after the kind of actions they help perform: physical and
cognitive. Physical and cognitive affordances help users perform physical and
cognitive actions, respectively. We agree with Norman that these two kinds of
affordance are not the same. They are essentially orthogonal concepts, but we
think they both play very important roles. The reason for our giving them new
names is to provide a better match to the kinds of actions they help users make
during their cycle of interaction. A physical affordance is a design feature
that helps, aids, supports, or facilitates physically doing something, and a
cognitive affordance is a design feature that helps, aids, supports, or facilitates
thinking and/or knowing about something.
Norman (1999: 41) makes the point that symbols and constraints are not affordances.
Thus, wording in the label on a button, for example, is symbolic communication.
We agree, but under our definition, this is exactly what makes the wording work
as a cognitive affordance, something to help the user in knowing (e.g. knowing
what to click on). We see symbols, constraints, and conventions as essential
underlying forces that make cognitive affordances work, as Norman says, as 'powerful
tools for the designer' of cognitive affordances. As Norman further says, the
only way we know for sure if users share designers' perceptions of these symbols
and conventions is by usability data. Thus, and we think this is a point that
Norman particularly had in mind in his article, if a designer claims to have
'added an affordance' to the interaction design, that in itself says nothing
about usability.
In the POET tradition, we illustrate with a simple and ubiquitous non-computer device, a device for opening doors. The hardware store carries both round doorknobs and the lever type door handles. The visual design of both kinds conveys a cognitive affordance helping users think or know about usage through the implied message their appearance gives to users: 'This is what you use to open the door'. Again, we agree with Norman in noting that the message is implicit and there is nothing intrinsic in the appearance of a doorknob that necessarily conveys this information. The doorknob is an easily recognized cultural convention because most people understand it as a symbol. On another planet, it could be perceived as mysterious and confusing. That is why a cognitive affordance is a perceived affordance. In this context, a doorknob is an excellent cognitive affordance because almost all users do share the same cultural convention. Door operation devices also provide physical affordance, to help users do the opening and closing - some better than others. For example, many users prefer the lever type over a round knob because the lever is easier to use with slippery hands or by an elbow when the hands are full.
Sometimes the physical affordance to help a user open a door is provided by
the door itself; people can open some swinging doors by just pushing on the
door. In such cases designers often help users by installing, for example, a
brass plate to show that one should push and where to push. Even though this
plate might help avoid handprints on the door, it is a cognitive affordance
and not a real physical affordance, because it adds nothing to the door itself
to help the user in the physical part of the pushing action. Sometimes the word
'Push' is engraved in the plate to augment the clarity of meaning of the plate
as a cognitive affordance.
Similarly, a swinging door must sometimes be opened by pulling. The door itself
does not usually offer sufficient physical affordance for the pulling action,
so a pull handle is added. A pull handle offers both physical and cognitive
affordance, providing a physical means for pulling as well as a visual indication
that pulling is required.
Norman makes it clear (1999: 39) that he was referring to perceived (cognitive) affordance in his POET book and he characterizes our view of cognitive affordance very well: 'When you first see something you have never seen before, how do you know what to do? The answer, I decided, was that the required information was in the world: the appearance of the device could provide the critical clues required for its proper operation'. When Norman later says that affordances play a relatively minor role in the world of screen-based systems (1999: 39), he clearly is talking about physical affordances (and the statement is true only if one is not concerned with Fitts' law, physical disabilities, or different kinds of interaction devices). It is quite the opposite case for cognitive affordances: cognitive affordances are one of the most significant usability-oriented features in present-day GUI-based systems. They are the key to answering Norman's question: 'How do you know what to do?' And, yes, the design of cognitive affordances does depend greatly on cultural conventions as a common base for communicating the meaning of visual cues from designer to user.
Continuing in the POET tradition of non-computer examples, I have had many different kinds of wine bottle openers, with a wide range of effectiveness. One particularly poor design, because of its crude mechanical operation, more often than not manages to crumble the cork, leaving bits of cork unappetizingly bobbing in the newly liberated libation. Although this device is easy to use, its design does not offer good physical affordance, in that it does not help the user very well in doing the physical task for which it was intended.
In contrast I have a friend whose cork puller is marvelously efficient and
reliably effective at removing the cork intact. The problem with this device,
though, is that its proper use is anything but obvious to me, so much so that
I have to ask again, each time I visit, how to operate it. This device offers
excellent physical affordance to help in doing the task, making it a good design
for an expert frequent user such as a wine steward. However, it does not offer
good cognitive affordance for helping intermittent and first-time users know
how to use it. There is not necessarily, as Norman points out, any connection
between the two kinds of affordance in a given design.
Soon: more about pereceptual affordance.
Paper on affordances, to appear in BIT.
Paper on affordances, Word version (temporary)