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The user interface, in the industrial design field of human–machine interaction, is the space where interaction between humans and machines occurs. The goal of this interaction is effective operation and control of the machine on the user's end, and feedback from the machine, which aids the operator in making operational decisions. Examples of this broad concept of user interfaces include the interactive aspects of computer operating systems, hand tools, heavy machinery operator controls, and process controls. The design considerations applicable when creating user interfaces are related to or involve such disciplines as ergonomics and psychology.
A user interface is the system by which people (users) interact with a machine. The user interface includes hardware (physical) and software (logical) components. User interfaces exist for various systems, and provide a means of:
Generally, the goal of human-machine interaction engineering is to produce a user interface which makes it easy (self exploratory), efficient, and enjoyable (user friendly) to operate a machine in the way which produces the desired result. This generally means that the operator needs to provide minimal input to achieve the desired output, and also that the machine minimizes undesired outputs to the human.
With the increased use of personal computers and the relative decline in societal awareness of heavy machinery, the term user interface is generally assumed to mean the graphical user interface, while industrial control panel and machinery control design discussions more commonly refer to human-machine interfaces.
Other terms for user interface include human–computer interface (HCI) and man–machine interface (MMI).
To work with a system, users have to be able to control and assess the state of the system. For example, when driving an automobile, the driver uses the steering wheel to control the direction of the vehicle, and the accelerator pedal, brake pedal and gearstick to control the speed of the vehicle. The driver perceives the position of the vehicle by looking through the windshield and exact speed of the vehicle by reading the speedometer. The user interface of the automobile is on the whole composed of the instruments the driver can use to accomplish the tasks of driving and maintaining the automobile. Interactive products must constantly be designed to support the way humans interact with the world and information.
There is a difference between a user interface and an operator interface or a human–machine interface.
In science fiction, HMI is sometimes used to refer to what is better described as direct neural interface. However, this latter usage is seeing increasing application in the real-life use of (medical) prostheses—the artificial extension that replaces a missing body part (e.g., cochlear implants).
In some circumstance computers might observe the user, and react according to their actions without specific commands. A means of tracking parts of the body is required, and sensors noting the position of the head, direction of gaze and so on have been used experimentally. This is particularly relevant to immersive interfaces.
The design of a user interface affects the amount of effort the user must expend to provide input for the system and to interpret the output of the system, and how much effort it takes to learn how to do this. Usability is the degree to which the design of a particular user interface takes into account the human psychology and physiology of the users, and makes the process of using the system effective, efficient and satisfying.
Usability is primarily a characteristic of the user interface, but is also associated with the functionalities of the product and the process to design it. It describes how well a product can be used for its intended purpose by its target users with efficiency, effectiveness, and satisfaction, also taking into account the requirements from its context of use.
The IBM Sage System of the 1950s was one of the first systems where the user actually interacted with the computer. Lights and switched were manually operated in order to obtain data by interacting with the system. Edward Tufte believed that information needed to be displayed in a clear and appealing manner.
In computer science and human–computer interaction, the user interface (of a computer program) refers to the information (such as graphic, text and sound) the program presents to the user, and the control sequences (such as keystrokes with the computer keyboard, movements of the computer mouse, and selections with the touchscreen) the user employs to control the program.
Direct manipulation interface is the name of a general class of user interfaces that allow users to manipulate objects presented to them, using actions that correspond at least loosely to the physical world.
Currently (as of 2009[update]) the following types of user interface are the most common:
User interfaces that are common in various fields outside desktop computing:
Other types of user interfaces:
The history of user interfaces can be divided into the following phases according to the dominant type of user interface:
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There are many principles and layers of design to an effective user interface. There is a communication system that consists of making it clear to the user what computer functions can and cannot occur, actual feedback that tells the user what is going on, and necessary consistency within every computer action.
Good user interface design is about getting a user to have a consistent set of expectations, and then meeting those expectations. Consistency can be bad if not used for a purpose and when it serves no benefit for the end user, though; like any other principle, consistency has its limits.
Consistency is one quality to trade off in user interface design as described by the cognitive dimensions framework. In some cases, a violation of consistency principles can provide sufficiently clear advantages that a wise and careful user interface designer may choose to violate consistency to achieve some other important goal.
First, the controls for different features should be presented in a consistent manner so that users can find the controls easily. For example, users find it difficult to use software when some commands are available through menus, some through icons, some through right-clicks, some under a separate button at one corner of a screen, some grouped by function, some grouped by “common”, some grouped by “advanced.” A user looking for a command should have a consistent search strategy for finding it. The more search strategies a user has to use, the more frustrating the search will be. The more consistent the grouping, the easier the search. The principle of monotony of design in user interfaces states that ideally there should be only way to achieve a simple operation, to facilitate habituation to the interface.
Second, there is the "principle of least astonishment". Various features should work in similar ways. For example, some features in Adobe Acrobat are "select tool, then select text to which apply." Others are "select text, then apply action to selection." Commands should work the same way in all contexts.
Third, consistency counsels against user interface changes version-to-version. Change should be minimized, and forward-compatibility should be maintained. Generally, less mature software has fewer users who are entrenched in the status quo. Older, more broadly used software must more carefully hew to the status quo to avoid disruptive costs. For example, the change from the menu bars of Microsoft Office 2003 to the ribbon toolbar of Microsoft Office 2007 caused mixed reactions. The new interface caused rejection among advanced users, who reported losses in productivity, while average users reported improved productivity  and a fairly good acceptance. A usual solution in providing a new user interface is to provide a backwards-compatibility mode, so that a product's most intensive users are not forced to bear the costs of the change. A second strategy is to introduce big changes in small increments, so that an overall redesign can be achieved without breaking consistency and providing user feedback at any single step.
Two words are used in UI design to describe the different ways in which a user can utilize a product. Modality refers to several alternate interfaces to the same product, while mode describes different states of the same interface.
A modality is a path of communication employed by the user interface to carry input and output. Examples of modalities:
The user interface may employ several redundant input modalities and output modalities, allowing the user to choose which ones to use for interaction.
A mode is a distinct method of operation within a computer program, in which the same input can produce different perceived results depending of the state of the computer program. For example, caps lock sets an input mode in which typed letters are uppercase by default; the same typing produces lowercase letters when not in caps lock mode. Heavy use of modes often reduces the usability of a user interface, as the user must expend effort to remember current mode states, and switch between mode states as necessary.
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