Usability is a quality of the user interface that refers to how easily and efficiently a user can perform a task using it. There are some principles that designers should incorporate into their design to maximize usability. These principles are divided into three broad categories:
Learnability
Flexibility
Robustness
Flexibility is the availability of multiple interaction methods between the user and the system. Following are the flexibility principles that support usability.
Dialog is the communication taking place between the user and the system. It can be of the following types:
User pre-emptive dialog, which the user initiates.
System pre-emptive dialog, which the system initiates.
The dialog initiative principle states that user pre-emptive dialog should be maximized and minimized to support flexibility. In other words, the user should control dialog flow that is free from system constraints. However, this is not entirely achievable because sometimes, the system needs to permit certain dialogs for security and safety reasons.
A thread is a part of a dialog which is related to a single task. The multi-threading principle allows multiple threads or tasks to run at the same time. Multi-threading can be either:
Concurrent: This allows simultaneous execution of multiple tasks. For example, editing a document on a word processor while it also increments and displays the word count.
Interleaved: This restricts the dialog to a single task at a time but has a temporal gap between multiple tasks. For example, users can have multiple documents open on a word processor but can only work on one document at a time.
Multi-threading also concerns
The principle of task migratability refers to efficiently transferring task execution control between the system and the user. Any running task should be easily passed over to the system or the user as needed. It also concerns the shared execution of tasks.
An example of task migratability is a spell-checker. It can be tiresome for a human to manually check for spelling mistakes in a document. Thus, the user can migrate this task to the system. However, the system does not recognize some words, such as names, which can result in errors. Therefore, spell-checking should be a cooperative process between the user and the system, allowed by the interface to maximize flexibility.
Substitutivity means that the user should be allowed to substitute equivalent input or output values. The interface aids this by permitting alternative ways of entering data and viewing results. For example, users can enter some length in either inches or centimeters.
Substitutivity is also concerned with representation multiplicity. This states that the output should be represented in multiple ways according to the need of the task. An example is the representation of time as either an analog or digital clock.
Customizability means that the interface should be modifiable by the user or the system. This modification is categorized into the following:
Adaptability: This refers to the modifications made by the user. This states that the user should be allowed to change the way of input and output according to their preference. For example, the user can change the taskbar's position in an interface.
Adaptivity: This refers to the modifications made by the system. This refers to automatic changes in the interface made by the system based on observed user expertise. For example, the system can detect whether the user is an expert or novice and display message boxes accordingly.
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