Notes on a Pattern Language for Interactive Usability

ABSTRACT

KEYWORDS

© 1997 Copyright on this material is held by the authors.

INTRODUCTION

An approach that has recently become prominent in object oriented software development is the use of patterns [1,5,7,9,10] to document the essentials of design solutions in a form that can be understood, learned, and applied to situations that are similar to existing cases but always subtly different. Many fields use patterns under various names. The pattern approach to design was first defined by the architect Christopher Alexander [2,3] twenty years ago. Similar structures appear in organizational behavior [14] as archetypes, in history of science [11] as paradigms, in military theory [15], in studies of mythology [13] again as archetypes, and even in a basic writing text [8] as templates. There is reason to believe from a cognitive point of view [4] that patterns might be a particularly effective way to organize complex information.

In interaction design, templates [6] are similar to patterns and have been proposed for the same purpose. The important advance needed to move from exemplars and templates to patterns seems to be a vocabulary that is specific to design for usability, simple enough to learn, and with a broad enough range of application. This paper suggests a vocabulary derived from the usability literature [12] and proposes a classification using the vocabulary.

PATTERNS AND PATTERN LANGUAGES

Name: A name for the pattern

Example: Window Place

Context: A context for the design problem

Example: Design of a residential room

Forces: Forces which require resolution

Example: People want to sit and also be in daylight.

Problem: A problem growing from the forces

Example: If all seating is away from the windows, then these forces are not resolved, and people will always be dissatisfied in one way or the other.

Solution: A known solution, proven in practice

Example: Build seating into the window -- the traditional window seat.

A pattern language is a collection of patterns that can solve all the problems in a particular domain. It may include a method for connecting patterns into whole "architectures" for the domain. (Less ambitiously, a "pattern system" [5] covers only parts of a domain.)

USABILITY PATTERNS

• Learnable
• Memorable
• Efficient
• Reliable
• Flexible
• Automated
• Understandable
• Subjectively Satisfying

To begin combing this limited vocabulary into sentences, we rate the importance of each. For this paper we rate each as high, medium, or low, although more fine-grained numeric scales are also effective. The combination of forces (ratings) in the context of a project defines the problem. For example, if a system has to be very easy to learn (Learnable) and very fast in use (Efficient), then the designer has a problem to solve because the two priorities are likely to conflict. As another example, if Learnable is rated high and Understandable also high, then the problem is easier to solve, and indeed there is a known solution – providing the user with a good conceptual model. Because there can be only a limited number of combinations of high ratings on the limited list of attributes, we can hope for known solutions to exist for many of them and for a focused search to uncover many others.

TYPES OF USABILITY PATTERNS

Simple Patterns

Name: "Fast and furious"

Context: GUI supporting variable, high-skill work with improvised tasks, as in a graphic editor

Forces: Efficiency is most important by far.

Problem: Support very fast manipulations.

Solution: Minimize action at the articulatory level through button bars, toolboxes, accelerators. This pattern can be observed in almost any modern word processor or graphic editor.

Intrinsic Patterns

Name:"Airport passenger"

Context: An interactive system requiring high skill performance on first encounter

Forces: Efficiency, reliability, and immediate learning of particulars are all required.

Problem: Balanced satisfaction of all requirements

Solution: Standardize components and procedures, leaving only unavoidable variation case to case. This solution can be seen in airports around the world, and in the consistency of GUI elements across platforms.

Circumstantial Patterns

Name:"Nouveau Vendo" (The whimsical name is from a configuration of vending machines with a single detached money insertion device for a set of machines. One user searched each machine unsuccessfully for a way to insert money and finally had to call for help.)

Context: External constraints require a system to be replaced by a new version with certain new features.

Forces: An overlearned behavior must be modified in a situation that is very similar to the learning situation.

Problem: Avoid capture or concept errors arising from overlearned or automatic behavior.

Solution: Make two systems either very different or completely consistent. Avoid almost consistent designs. Don't let small variations creep in through late design changes or design compromises.

FUTURE OPPORTUNITIES

• Look at combinations of usability attributes to see which are difficult to design for and which are easy. Look for example applications of the combinations.
• Analyze existing computer applications to see what usability patterns have implicitly driven the design.
• Look for usability patterns in the real world. When a system has usability problems, try the idea that there are forces that are difficult to resolve before assuming that the designer was careless.
• Document new patterns. They have most value when they are shared. Often all that is required is a hint for others to gain insights into puzzling situations.

ACKNOWLEDGEMENT

REFERENCES