Chapter 2
Basic Concepts
The vocabulary that everything else in usability and UX is built on: human-centred quality, the three criteria of usability, and how it differs from user experience.
Core definitions
Human-centred design (HCD)
+Human-centred quality
+Usability
+Effectiveness
+Efficiency
+Satisfaction
+User experience (UX)
+Accessibility
+Avoidance of harm from use
+Interactive system
+User interface & UI element
+The three criteria of usability
Supports what users need to do — and they can figure out how.
Complete tasks quickly, without having to think too much.
Meets users' expectations and is pleasant to use.
Usability vs. user experience
Usability lives inside actual use (effectiveness, efficiency, satisfaction). User experience wraps around it — expectations before, satisfaction during, fulfilment after.
Reading it: A usability problem at checkout hurts both UX and usability. Poor-quality flowers delivered, high prices, or a bad in-store visit hurt only UX — usability is only about using the system itself.
What counts as an "interactive system"
The definition is broader than it seems. It covers systems that take two-way input (websites, apps, ATMs) and systems that only display information one-way — like arrival/departure boards at an airport or train station. As long as a person uses it to reach a goal, it's an interactive system and can be evaluated for usability. The definition also includes packaging, user documentation, online help, support and training.
One product that makes every foundation concept click: the hotel-booking app “StayEasy”
Using a single product — the StayEasy booking app — to string together every term in Foundations & quality, following the natural flow from “what the product is” to “what quality means for the user.”
Each highlighted word is a term from Section 1 — compare it with the definition card above.
StayEasy is an interactive system
StayEasy is an interactive system: not just the app, but the software, servers, help documentation and support around it. Guests see and operate it through the user interface — all the screens, information and controls. Each basic component like a “Book” button, a date field, or a “Remember me” checkbox is a user interface element.
The team chooses a human-centred approach
Rather than guessing, the team applies human-centred design (HCD): build an explicit understanding of the users, their goals and context, involve them throughout, and iterate on feedback until requirements are met. The overall goal of this approach is human-centred quality — quality seen from the user's point of view, made of the four pieces in parts C and D.
Usability — and the three criteria behind it
Usability is how far StayEasy can be used effectively, efficiently and with satisfaction in a specified context. It rests on three pillars:
The guest gets the job done: the app has a cancel button and guests can find it. If only 5/100 new guests manage to cancel → an effectiveness problem.
Done fast, with little effort: cancelling takes 2 steps. If everyone can cancel but only through 8 fiddly steps → an efficiency problem.
It feels pleasant to use: guests praise the clean layout. If a guest says booking “takes forever” → a satisfaction problem.
The three criteria influence each other: low efficiency can drag satisfaction down; frustration can make a guest quit mid-task, which hurts effectiveness.
User experience and the other two pieces of quality
User experience (UX) is broader than usability: it also covers how the guest feels before use (the expectation set by a StayEasy ad), during use (the satisfaction part, which overlaps with usability) and after use (does the real room match what the app promised?). Usability considers satisfaction only during use; UX spans the whole journey.
A telling contrast: a bug at checkout hurts both usability and UX. But if the real room looks worse than its photos, that hurts only UX — not usability, because usability is only about using the app.
The last two pieces of human-centred quality:
A blind guest using a screen reader can still book a room; text is large enough with sufficient contrast (per WCAG).
The app requires explicit confirmation before booking a non-refundable rate, preventing accidental financial harm.
Putting it together: StayEasy is an interactive system that guests operate through a user interface made of many UI elements. The team uses human-centred design to aim for human-centred quality. That quality = usability (effective + efficient + satisfying) + user experience (before–during–after) + accessibility (usable by everyone) + avoidance of harm (does no damage). Miss one piece and the product hasn't reached quality for the user.