Usability Engineering

Overview

Why exactly is an interactive system hard to use, and whose job is it to find out?

The professional discipline that studies and tests the usability of interactive systems, drawing on computer science and psychology to identify why something is hard to use. It’s distinct from UX or interaction design in where it focuses effort: a usability engineer concentrates on the research and testing phase — surfacing problems — while a UX or interaction designer focuses on solving them. In practice the two roles work together throughout Iterative Design‘s design→prototype→evaluate cycle.

The professional discipline that studies and tests the usability of interactive systems, drawing on computer science and psychology to identify why something is hard to use.

The five usability attributes

Nielsen‘s framework names five qualities a usability evaluation should check a design against:

• Learnability — how quickly a new user can become proficient
• Efficiency — how quickly an experienced user can complete tasks
• Memorability — how well infrequent users retain proficiency between visits
• Errors — how often errors occur, how severe they are, and how easily they’re recovered from
• Satisfaction — how pleasant the system is to use, subjectively

These give concrete substance to what Usability Evaluation Methods‘s expert reviews, informal evaluations, and formal studies are actually testing for.

ISO 9241’s definition

The ISO 9241 standard (part 11) operationalizes usability as the effectiveness, efficiency, and satisfaction with which specified users can achieve specified goals in a specified context of use — a more formal, standards-body counterpart to Nielsen’s five attributes, used by usability engineers to ground interface requirements in an accepted definition rather than an informal one.

The ISO 9241 standard (part 11) operationalizes usability as the effectiveness, efficiency, and satisfaction with which specified users can achieve specified goals in a specified context of use — a more formal, standards-body counterpart to Nielsen’s five attributes, used by usability engineers to ground interface requirements in an accepted definition rather than an informal one.

Errors: slips vs. mistakes

A two-part taxonomy of human error, useful for diagnosing why an error happened rather than just that it did:

• A slip is the right goal, executed via the wrong action — typos, clicking the wrong menu item — typically a failure of automatic behavior rather than judgment.
• A mistake is a consciously deliberated but wrong goal in the first place — the user’s plan itself was flawed.

A mode error is a particularly dangerous slip subtype: the user acts correctly for the mode they believe they’re in, but the system is actually in a different one — a classic example is typing text expecting insertion, only to discover Caps Lock or an editor’s “replace” mode was on. This sharpens Interface Design Principles‘s fourth principle (helping customers avoid and recover from errors): preventing slips and preventing mistakes call for different design responses, since one is about reducing accidental wrong actions and the other is about catching a wrong plan before it’s acted on. See Modal vs. Modeless Interfaces for mode errors in full — why they happen, quasimodes as a mitigation, and design recommendations for avoiding them — and Error Prevention and Recovery for slip/mistake prevention, detection, and recovery techniques generally.

A mode error is a particularly dangerous slip subtype: the user acts correctly for the mode they believe they’re in, but the system is actually in a different one — a classic example is typing text expecting insertion, only to discover Caps Lock or an editor’s “replace” mode was on.

See Usability Evaluation Methods for the RITE method, a faster variant of the evaluation methods covered there and in Customer Research Methods.

Related Concepts

Principles

• Interface Design Principles
• Modal vs. Modeless Interfaces
• Error Prevention and Recovery

Processes

• Iterative Design
• Usability Evaluation Methods
• Customer Research Methods

Further reading

Jakob Nielsen’s Usability Engineering (Academic Press / Morgan Kaufmann, 1993, ISBN 978-0125184069 — commercially published, no stated open license) is the full-length book that coined the discipline this page describes, developing cost-effective usability methods and a stage-by-stage guide to which method fits which point in a development lifecycle, plus a dedicated treatment of international usability testing this page doesn’t cover.

Sources

Usability Engineering (Wikipedia) is this page’s sole source — the learnability/efficiency/memorability/errors/satisfaction framework, the ISO 9241 Part 11 definition, and the slip/mistake/mode-error taxonomy.