Two of the most important aspects of good design are discoverability and understanding. Discoverability: is it possible to even figure out what actions are possible and where and how to perform them? Understanding: What does it all mean? How is the product supposed to be used? What do all the different controls and settings mean?
Design is concerned with how things work, how they are controlled, and the nature of the interaction between people and technology.
...we must design our machines on the assumption that people will make errors.
Good design requires good communication, especially from machine to person, indicating what actions are possible, what is happening, and what is about to happen. Communication is especially important when things go wrong. [...] Designers need to focus their attention on the cases where things go wrong, not just on when things work a planned.
Human centered design [starts with] a good understanding of people and the needs the design is intended to meet. This understanding comes about primarily through observation, for people themselves are often unaware of their true needs, even unaware of the difficulties they are encountering. Getting the specification of the thing to be defined is one of the most difficult parts of the design, so much so that the HCD principle is to avoid specifying the problem as long as possible but instead to iterate upon repeated approximations. This is done through rapid tests of ideas, and after each test modifying the approach and the problem definition.
Experience (feeling, emotions, subjective stuff) is critical, for it determines how fondly people remember their interactions. [...] Cognition and emotion are tightly intertwined [with experience], which means that the designers must design with both in mind.
Five fundamental psychological concepts: affordances, signifiers, constraints, mappings, and feedback. There is a sixth principle that is perhaps the most important of all: the conceptual model of the system. It is the conceptual model that provides true understanding.
The presence of an affordance is jointly determined by the qualities of the object and the abilities of the agent that is interacting (emphasis mine). [... An] affordance is not a property. An affordance is a relationship
If an affordance or anti-affordance cannot be perceived, some means of signaling its presence is required.
Affordances determine what actions are possible. Signifiers communicate where the action should take place. We need both. [...] Good design requires, among other things, good communication of the purpose, structure, and operation of the device to the people who use it.
The signifier is an important communication device to the recipient, whether or not communication was intended.
Affordances represent the possibilities in the world for how an agent can interact with something. Signifiers are signals.
Signifiers signal things, in particular what actions are possible and how they should be done. Signifiers must be perceivable, else they fail to function.
In design, signifiers are more important than affordances, for they communicate how to use the design.
[...] afordances and signifiers are fundamentally important principles of good design.
The relationship between a control and its results is easiest to learn wherever there is an understandable mapping between the controls, the actions, and the intended result.
A device is easy to use when the set of possible actions is visible, when the controls and displays exploit natural mappings. The principles are simple but rarely incorporated into design. Good design takes care, planning, though, and an understanding of how people behave.
Feedback - communicating the results of an action - is a well known concept from the science of control and information theory.
Feedback must be immediate: even a delay of a tenth of a second can be disconcerting. If the delay is too long, people often give up, going off to do other activities. [...] Feedback must also be informative.
Poor feedback can be worse than no feedback at all, because it is distracting, uninformative, and in many cases irritating and anxiety-provoking. [...] Feedback is essential, but not when it gets in the way of other things, including a calm and relaxing environment.
Feedback has to be planned. All actions need to be confirmed, but in a manner that is unobtrusive. Feedback must also be prioritized, so that unimportant information is presented in an unobtrusive fashion, but important signals are presented in a way that does capture attention
A conceptual model is an explanation, usually highly simplified, of how something works. It doesn't have to be complete or even accurate as long as it is useful (emphasis mine)
Simplified models are valuable only as long as the assumptions that support them hold true. There are often multiple conceptual models of a product or device.
There is no need to understand the underlying physics or chemistry of each device we own, just the relationship between the controls and the outcomes.
The combined information available to us is the system image. When the system image is incoherent or inappropriate, then the user cannot easily use the device. If it is incomplete or contradictory, there will be trouble.
Good conceptual models are the key to understandable, enjoyable products: good communication is the key to good conceptual models.
The same technology that simplifies life by providing more functions in each device also complicates life by making the device harder to learn, harder to use. This is the paradox of technology, and the challenge for the designer.
