80/20 Rule

A high percentage of effects in any large system are caused by a low percentage of variables.1

The 80/20 rule asserts that approximately 80 percent of the effects generated by any large system are caused by 20 percent of the variables in that system. The 80/20 rule is observed in all large systemss, includeing those in economics, management, user interface design, quality control, and engineering, to name a few. The specific percentages are not important, as measures of actual systems indicate that the proportion of critical variables varies between 10 percent and 30 percent. The universality of the 80/20 rule suggests a link to normally distrubuted systems, which limits its application to variables that are influenced by many small and unrelated effects -- e.g., systems that are used by large numbers of people in a variety of ways. A few examples of the 80/20 rule include:2

80 percent of a product's usage involves 20 percent of its features.

80 percent of a town's traffic is on 20 percent of its roads.

80 percent of a company's revenue comes from 20 percent of the people.

80 percent of innovation comes from 20 percent of the people.

80 percent of progress comes from 20 percent of the effort.

80 percent of errors are caused by 20 percent of the components.

The 80/20 rule is useful for focusing resources and, in turn, relizing greater efficiencies in design. For example, if the critical 20 percent of a product's features are used 80 percent of the time, design and testing resources should focus primarily on those features. The remaining 80 percent of the features should be reevaluated to verify their value in the design. Similarly, when redesigning systems to make them more efficient, focusing on aspects of the system beyond the critical 20 percent quickly yields diminishing returns; improvements beyond the critical 20 percent will result in less substantial gains that are often offset by the introduction of errors or new problems into the system.

All elements in a design are not created equal. Use the 80/20 rule to assess the value of elements, target areas of redesign and optimization, and focus resources in an efficient manner. Noncritical functions that are part of the less-important 80 percent should be minimized or removed altogether from the design. When time and resources are limited, resist efforts to correct and optimize designs beyond the critical 20 percent, as such efforts yield diminishing returns. Generally, limit the application of the 80/20 rule to variables in a system that are influenced by many small and unrelated effects.

See also Cost-Benefit, Form Follows Function, Highlighting, Most Advanced Yet Acceptable, and Normal Distribution.

1 Also known as Pareto's Principle, Juran's Principle, and Vital Few and Trivial Many Rule.

2 The first recognition of the 80/20 rule is attributed to Vilfredo Pareto, an Italian economist who observed that 20 percent of the Italian people possessed 80 percent of the wealth. The seminal work on the 80/20 rule is Quality Control Handbook by Joseph M. Juran (Ed.), McGraw-Hill, 1951.

Graphical user interfaces conceal most of their funtions in drop-down menus (bottom image). This reduces the complexity of the display, but also makes frequently used functions more difficult to access. Identifying the critical 20 percent of the functions and making them readily available in toolbars solves the problem (top image).

Accessibility

Objects and environments should be designed to be usable, without modification, by as many people as possible.1

The principle of accessibility asserts that designs should be usable by people of diverse abilities, without special adaptation or modification. Historically, accessibility in design focused on accommodating people with disabilities. As knowledge and experience of accessible design increased, it became increasingly clear that many required "accommodations" could be designed to benefit everyone. There are four characteristics of accessible designs: perceptibility, operability, simplicity, and forgiveness. 2

Perceptibility is achievedd when everyone can perceive the design, regardless of sensory abilities. Basic guidelines for improving perceptibility are: present information using redundant coding methods (e.g., textual, iconic, and tactile); provide compatibility with assistive sensory technologies (e.g., ALT tags for images on the internet); and position controls and information so that seated and standing users can perceive them.

Operability is achieved when everyone can use the design, regardless of physical abilities. Basic guidelines for improving operability are: minimize repetitive actions and the need for sustained physical effort; facilitate use of controls through good affordances and constraints; provide compatibility with assistive physical technologies (e.g., wheelchair access); and position controls and information so that seared and standing users can access them.

Simplicity is achieved when everyone can easily understand and use the design, regardless of experience, literacy, or concentration level. Basic guidelines for improving simplicity are: remove unnecessary complexity; clearly and consistently present only relevant information and controls; provide clear prompting and feedback for all actions; and ensure that reading levels accommodate a wide range of literacy.

Forgiveness is achieved when designs minimize the occurrence and consequences of errors. Basic guidelines for improving forgiveness are: use good affordances and constraints (e.g., controls that can only be used the correct way) to prevent errors from occurring; use confirmations and warnings to reduce the occurrence of errors; and include reversible actions and safety nets to minimize the consequence of errors (e.g., the ability to undo an action).

See also Affordance, Forgiveness, Legibility, Normal Distributuion, and Readability.

1 Also known as barrier-free design and related to universal design and inclusive design.

2 The four characteristics of accessible designs are derived from W3C Web Content Accessibility Guidelines 1.0,1999; ADA Accessibility Guidelines for Buildings and Facilities, 1998; and Accessible Environments, Toward Universal Design by Ronald L. Mace, Graeme J. Hardie, and Jaine P. Place, The Center for Universal Design, North Carolina State University, 1996.

The large elevator has many features that make it more accessible than the small elevator: wide doors permit easy access; handrails help people mantain a standing position; two sets of controls are easily accessible from a seated position; controls redundantly coded with numbers, icons, and Braille; feedback is provided visually and aurally; and an emergency phone system offers access to special ssistance.

-- Universal Priciples of Design, by William Lidwell, Kritina Holden, Jill Butler, Page 14-17.