Glossary

7 +/- 2

A theory proposed by cognitive psychologist George A. Miller that the channel capacity (the maximum number of separate pieces of information that can be processed) of a human when performing perceptual tasks is between 5 and 9 (7 +/- 2). This is equivalent to about 2.5 bits of information. Miller felt that this might be due to some unknown underlying human mechanism.

7 Stages of Action

A 7-stage model proposed by Norman for how humans accomplish their desired goals with objects:

Goals

1. Goals - what you want to do

Execution

2. Intention to act - translation of goals into a set of tasks
3. Sequence of actions - ordering of tasks in proper sequence
4. Execution of actions - performance of action sequence tasks

Evaluation

5. Perception of results - monitoring of situation after execution
6. Interpretation of perception - evaluation of situation
7. Evaluation of interpretation - comparison of situation to desired goals

For example:

Goals

1. You want to shoot a bottle with a pistol.

Execution

2. You intend to aim the pistol at the bottle and fire.
3. You intend to pick up the pistol, turn the safety off, hold it properly, use the sight to aim at the target, and pull the trigger to fire.
4. You execute this action sequence.

Evaluation

5. You hear a loud report, feel the pistol recoil, see and smell smoke, and see that the bottle has shattered.
6. You interpret these as meaning that the pistol has shot the bottle.
7. You deem this a success; your goal of shooting the bottle has been accomplished.

affordance

A property of an object that indicates how this object can be used.

The grip of a pistol affords holding. The barrel sight affords aiming. The trigger affords pulling (and shooting).

In Norman's usage, affordance is subjective, taking into account the goals, plans, values, beliefs, and interests of the human user of the object. For this reason it is often called perceived affordance.

A subjective (perceived) Norman affordance of a pistol is holding, aiming, and shooting. An objective affordance might be to hammer a nail into a piece of wood, or to use as a paperweight on a desk, or to hang by a wire from your living room ceiling as a mobile.

conceptual model

A mental image of how an object works.

There are two types of conceptual models:

- design model - the designer's model
- user's model - the user's model that develops through interaction with the system

Ideally, the design and user's models are identical. Alas, this is not always the case; hence the need for usability analysis.

The design and user's models for a pistol are very similar. Both understand fully that the pistol is made to shoot bullets at targets. The models diverge to some extent when it comes to the details: how to load the bullets, how to use the safety, how to minimize recoil, etc.

constraint

A property of an object that limits how this object can be used.

The trigger guard of a pistol limits the number of fingers that can be used to pull the trigger to one (or perhaps two). The physical relationship between the grip and the trigger limits which finger(s) can be used to pull the trigger (e.g., not the thumb). The engineering of the trigger limits the direction it can be moved (backward).

feedback

Information communicated back to users about actions they have taken.

Feedback shows users the effects of their actions. Setting a pistol safety switch to its on or off position might produce an audible feedback click or a palpable drop into a slot. Loading a bullet properly in the breach might produce an audible and palpable snap. Shooting produces an audible report and palpable recoil.

Without clear feedback, users can feel lost, clueless, unsure how to proceed.

gulf of evaluation

How hard a user must work to assess the results of an action performed on an object.

If your goal is to shoot a target with a pistol, there is no appreciable gulf of evaluation in assessing whether the pistol has been fired, but there can be a sizeable gulf of evaluation in assessing whether the target has been hit (if it is small, far away, etc.).

gulf of execution

The difference between a user's intentions and the object's allowable actions.

If your intention is to shoot a pistol, there is no appreciable gulf of execution (assuming that the pistol is functional, loaded, etc.); you simply pick up the pistol and pull the trigger. If your intention is to shoot three bullets in one second, unless you are using an automatic weapon or you have extraordinarily fast reflexes, the gulf of execution is huge.

mapping

The relationship between the controls of an object (knobs, switches, levers, buttons, pedals, keys, etc.) and what they can be used for.

Consider the mappings involved in hitting a target with a pistol: 1) cupping the grip in the palm to hold the pistol properly; 2) pointing the barrel toward the target; 3) using the sight to aim; 4) pulling the trigger back to shoot.

See also: natural mapping.

natural design

Design that makes use of natural mappings.

natural mapping

A mapping that takes advantage of physical analogies and cultural standards, making it easy to understand.

The shape of a pistol and everyday laws of physics make it clear that the barrel of a pistol should be pointed at the intended target. The engineering of the trigger guard (lots of space in front of the trigger, very little behind) make it clear that the trigger should be pulled backwards to shoot. The safety switch, on the other hand, is not naturally mapped, and for this reason users must work at learning how to use it properly.

psychology

The study of the human (and animal) mind.

The original title of Norman's book was The Psychology of Everyday Things. He really liked this, especially because of its acronym: POET. Alas, designers ended up steering clear of the book due to the "psychology" in its title, and bookstores generally misplaced it in the Psychology section next to the dreary "How To ..." books, all of which reduced sales. So he compromised and renamed it The Design of Everyday Things.

psychopathology

The study of mental disorders.

Norman's use of the term as a chapter title, "The Psychopathology of Everyday Things," is a tongue-in-cheek allusion to Freud's book, "The Psychopathology of Everyday Life."

system image

An object and all its supplementals: reference manuals, instructions, diagrams, labels, icons, etc.

The system image of a pistol is the pistol itself, any text/labels/icons on the pistol, the user manual, associated diagrams, etc.

visibility

The degree to which an object's intended usage is visible to the user.

The shape of a pistol (grip, barrel, trigger) makes the way it is intended to be held quite visible. The location and movement of the trigger guard and trigger make the act of pulling the trigger reasonably visible. The long straight barrel with a hole in one end makes the act of shooting the pistol somewhat, though by no means clearly, visible.

(definitions courtesy of Professor Oyzon)