Understanding user interface design rules - Part 1: Task analysis and conceptual model

In addition to being focused on users' tasks, a conceptual model should be as simple as possible. Simpler means fewer concepts. The fewer concepts a model has for users to master, the better, as long as it provides the required functionality. Less is more, provided that what is there fits well with users' goals and tasks.

For example:

• In a To-Do List application, do users need to be able to assign priorities of 1 – 10 to items, or are two priority levels - low and high - enough?

• Does a Search function need to allow users to enter full Boolean expressions? If it allowed that, would a significant number of people use it? If not, leave it out.

• Does a ticket machine in a train station need to be able to offer tickets for train routes other than the routes that this station is on?

In most development efforts, there is pressure to add extra functionality "in case a user might want it." Resist such pressure unless there is considerable evidence that a significant number of potential customers and users really need the extra functionality. Why? Because every extra concept increases the complexity of the software. It is one more thing users have to learn. But actually it is not just one more thing.

Each concept in an application interacts with most of the other concepts, and those interactions result in more complexity. Therefore, as concepts are added to an application, the application's complexity grows not just linearly, but multiplicatively (see Fig. 11.1).

Consistency
The consistency of an interactive system strongly affects how quickly its users progress from controlled, consciously monitored, slow operation to automatic, unmonitored, faster operation (Schneider & Shiffrin, 1977). The more predictable the operation of a system's different functions, the more consistent it is.

In a highly consistent system, the operation of a particular function is predictable from its type, so people quickly learn how everything in the system works and its use quickly becomes habitual. In an inconsistent system, users cannot predict how its different functions work, so they must learn each one anew, which slows their learning of the overall system and keeps their use of it a controlled, attention-consuming process.

Interactive systems can be consistent or inconsistent on at least two different levels: the conceptual level and the keystroke level. Consistency at the conceptual level is determined by the mapping between the objects, actions, and attributes of the conceptual model (see above). Do most objects in the system have the same actions and attributes, or not?

Consistency at the keystroke level is determined by the mapping between the conceptual actions and the physical movements required to execute them. Are all conceptual actions of a certain type initiated and controlled by the same physical movements, or not?