In this article
Fundamentals
What is the difference between UX design and UI design?
User experience (UX) covers every part of a person's dealings with a company, its services and its products, while the user interface (UI) is one part of it. Nielsen Norman Group's definition of user experience (1998) says exactly that, and gives the example of a movie review site whose search is perfect but whose database lacks independent films: the interface is fine and the experience is not.
Why it is asked: it shows whether you design for outcomes or only for screens, which any interview loop probes early. Read the full explanation: Nielsen Norman Group, The Definition of User Experience.
What is usability, and what are its components?
Usability is a quality attribute that measures how easy a user interface is to use. Nielsen Norman Group's Usability 101 (January 2012) names five components: learnability, how easily users do basic tasks the first time; efficiency, how fast they work once they have learned the design; memorability; errors, meaning how many, how severe and how easily fixed; and satisfaction.
Why it is asked: the five components give you a checklist for judging any design out loud. Read the full explanation: Nielsen Norman Group, Usability 101.
What are the ten usability heuristics?
A heuristic is a broad rule of thumb for judging an interface. Nielsen Norman Group's list (first published April 1994) has ten: visibility of system status; match between the system and the real world; user control and freedom; consistency and standards; error prevention; recognition rather than recall; flexibility and efficiency of use; aesthetic and minimalist design; help users recognize, diagnose and recover from errors; and help and documentation.
Why it is asked: a heuristic evaluation, in which a reviewer inspects a design against these rules, is a fast review you can run with no users. Read the full explanation: Nielsen Norman Group, 10 Usability Heuristics.
Which heuristic does an undo button satisfy?
An undo button satisfies "user control and freedom": people make mistakes and need a clearly marked way out of an unwanted state without a long dialogue. A confirmation dialog before deleting is a different heuristic, error prevention, because it stops the mistake instead of repairing it. Naming the pair and the difference is the strong answer.
Why it is asked: it tests whether you can map a real feature to a named principle. Read the full explanation: Nielsen Norman Group, 10 Usability Heuristics.
Process
What is the Double Diamond design process?
The Double Diamond is the Design Council's model of the design process, which dates from 2003, with four phases. Discover and define work on the problem; develop and deliver work on the solution. Discover means understanding the problem by talking with the people affected, define frames the challenge from that insight, develop tries different answers, and deliver tests small, drops what fails and improves the rest. Each diamond first widens to explore, then narrows to choose, and what you learn can send you back to the start of the diamond, as the Design Council describes.
Why it is asked: it shows whether you can describe a process without pretending it is a straight line. Read the full explanation: Design Council, The Double Diamond.
Step-by-step description
- Understand the problem by talking to people.
- Frame the challenge from the insight.
- Try different answers to the defined problem.
- Test small, drop what fails, improve the rest.
- Discover and define work on the problem.
- Develop and deliver work on the solution.
What is the difference between a user journey and a user flow?
A user journey is a high-level, scenario-based sequence of steps a person takes to reach a goal, often across several channels and over time. A user flow is a granular set of interactions inside one product for a short task. Nielsen Norman Group (April 2023) adds that journeys are drawn as journey maps, which also capture thoughts and feelings, and flows as flow charts or wireflows, which do not.
Why it is asked: the two are often confused, and choosing the right one shows scope judgement. Read the full explanation: Nielsen Norman Group, User Journeys vs. User Flows.
How do you decide where to place buttons and how big to make them?
Fitts's law is a model of pointing that says the time to reach a target depends on its size and its distance. Nielsen Norman Group's summary (July 2022) gives T = a + b·log₂(2D/W), with D the distance, W the target width, and a and b constants for the pointing device. Make targets large, put the primary action near the last field of a form, and keep controls used in sequence close together without crowding them.
Why it is asked: it turns a vague taste question into a reasoned layout decision. Read the full explanation: Nielsen Norman Group, Fitts's Law and Its Applications in UX.
Knowing it is half the job. Saying it is the other half.
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How do you choose a UX research method?
Place the question on two axes. Attitudinal methods capture what people say and behavioral methods capture what they do; qualitative methods answer "why" and "how to fix", and quantitative methods answer "how many" and "how much". Nielsen Norman Group (July 2022) places surveys as attitudinal and quantitative, focus groups as attitudinal and qualitative, A/B testing as behavioral and quantitative, and usability testing as qualitative and leaning behavioral.
Why it is asked: it shows you pick a method to fit a question rather than by habit. Read the full explanation: Nielsen Norman Group, When to Use Which UX Research Methods.
What is a usability test?
A usability test has three components, per Nielsen Norman Group's Usability Testing 101 (December 2019): a facilitator who guides the participant and asks follow-up questions, realistic tasks for the participant to perform, and a participant who is a realistic user, often asked to think aloud while working. You watch what the participant does, not only what they say.
Why it is asked: it checks that you have run one, not just read about one. Read the full explanation: Nielsen Norman Group, Usability Testing 101.
How many users do you need for a usability test?
Five per round is the classic answer. Nielsen (March 2000) models the problems found by n users as N(1 − (1 − L)^n), where L, typically 31%, is the share one user finds. That gives 31.0% for one user, 84.4% for five and 99.6% for fifteen1. His advice is three studies of five users with a redesign between them, not one large study. The five holds for a qualitative test of one group of similar users: test each distinct user group separately, and a quantitative study needs about twenty users (Nielsen, March 2000).
Why it is asked: it separates a rule you can derive from a number you memorised. Read the full explanation: Nielsen, the five-user usability testing article.
Text description of the diagram
- One user finds 31.0% of the usability problems and five users find 84.4%.
- Fifteen users find 99.6% of the problems under the same model.
Why does the fifth user add less than the second?
Each user mostly re-finds problems earlier users already found. Under the same model the second user adds 21.4 percentage points (from 31.0% to 52.4%), while a sixth user adds only 4.8 (from 84.4% to 89.2%)1. Five users leave about 15.6% of problems unfound, and a second round of five leaves about 2.4%. The model assumes a typical L of 31%, which varies by project.
Why it is asked: it tests whether you understand diminishing returns and not only the rule. Read the full explanation: Nielsen, the five-user usability testing article.
What is card sorting, and when is it used?
Card sorting is a method in which participants group labelled cards by whatever logic makes sense to them, which shows how they categorise content. In an open sort they create their own groups and names, the most common type; in a closed sort they use categories you set. Nielsen Norman Group (February 2024) uses it to shape information architecture and generally prefers tree testing to validate navigation.
Why it is asked: it tests whether you know how a navigation structure is evidenced. Read the full explanation: Nielsen Norman Group, Card Sorting.
When would you use A/B testing, and when not?
A/B testing is a research method that splits live visitors between a control and a variant and shows which better meets goals set in advance. Nielsen Norman Group (August 2024) says it falls short on low-traffic pages, where reaching significance can take thousands of visits, when several changes are bundled, and for understanding motives, because it shows what people do but not why. Pair it with qualitative research.
Why it is asked: it shows you know a test's limits as well as its mechanics. Read the full explanation: Nielsen Norman Group, A/B Testing 101.
Accessibility and delivery
What is WCAG, and what are its four principles?
The Web Content Accessibility Guidelines (WCAG) are the W3C's technical standard for making web content accessible. Version 2.2 became a W3C Recommendation in October 2023; the current edition is dated 12 December 2024. Its four principles are perceivable, operable, understandable and robust, and it has three conformance levels: A, double-A and triple-A, from lowest to highest.
Why it is asked: the middle level, double-A, is the one most teams aim for, so you should know what it means. Read the full explanation: W3C, WCAG 2.2.
What contrast ratio does text need for accessibility?
Normal text needs a contrast ratio of at least 4.5 to 1 against its background, and large-scale text needs at least 3 to 1. That is success criterion 1.4.3, Contrast (Minimum), in WCAG 2.2. A brand colour pair that looks fine on your screen can fail it, so you check the ratio with a tool rather than by eye.
Why it is asked: it separates designers who have shipped accessible work from those who list it as a value. Read the full explanation: W3C, WCAG 2.2.
How big should a tap or click target be?
At least 24 by 24 Cascading Style Sheets (CSS) pixels at level double-A. That is success criterion 2.5.8, Target Size (Minimum), explained in W3C's Understanding page (updated May 2026). A smaller target can pass if spacing keeps a 24-pixel circle clear of other targets, if an equivalent control exists, or if the target is inline in text.
Why it is asked: it tests whether you know a number and its exceptions. Read the full explanation: W3C, Target Size (Minimum).
Preparing for the round
How do you present a portfolio case study?
Tell it as the Double Diamond told in your own project: the problem you found, how you framed it, the options you tried, and what testing changed. Spend the most time on one decision you reversed after evidence, and say the number or observation that reversed it. Pair it with a short self-introduction so the case study has context.
Why it is asked: it shows judgement and your own part in a team result. Read the full explanation: Design Council, The Double Diamond.
How do you answer behavioural questions in a UX interview?
Use the situation, task, action, result (STAR) structure and keep the result measurable or observable. Disagreements with developers or stakeholders are common prompts; the strong answer states the user evidence you brought rather than a taste argument. The STAR method and the behavioral interviews guide cover the structure in full.
Why it is asked: design is collaborative, and the loop tests that as much as craft. Read the full explanation: behavioral interview guide.
How should I prepare for a UX designer interview?
Prepare two case studies you can tell in five minutes, a usability test you can describe step by step, and a one-line answer to each question above. Then rehearse the loop: this page is one of the business support and creative role guides, see how interview loops are structured, prepare questions to ask the interviewer, and for roles that touch code read the front-end developer questions.
Why it is asked: it shows how you study and how you work. Read the full explanation: interview loops.
The numbers behind the testing answers
| Test users | Share of problems found |
|---|---|
| 1 | 31.0% |
| 2 | 52.4% |
| 3 | 67.1% |
| 4 | 77.3% |
| 5 | 84.4% |
| 10 | 97.6% |
| 15 | 99.6% |
The row for five users is the one to remember: it leaves about 15.6% of problems for the next round1.
- Define UX as the whole experience and the interface as one part of it.
- Name the five components of usability and the ten heuristics, and map a feature to one.
- Choose a research method by question: what people do or say, why or how many.
- Five users per round find about 84% of problems; test in rounds, not once.
- Know the two WCAG numbers: 4.5 to 1 contrast and 24-pixel targets.
Sources
- Nielsen (2000), the five-user usability testing article — Nielsen Norman Group (Nielsen), 18 Mar 2000
- The Double Diamond — Design Council, 1 Jan 2003
- Web Content Accessibility Guidelines (WCAG) 2.2 — W3C, 12 Dec 2024
- Understanding Success Criterion 2.5.8: Target Size (Minimum) — W3C Web Accessibility Initiative, 11 May 2026
- 10 Usability Heuristics for User Interface Design — Nielsen Norman Group (Nielsen), 24 Apr 1994
- Usability 101: Introduction to Usability — Nielsen Norman Group (Nielsen), 3 Jan 2012
- Usability Testing 101 — Nielsen Norman Group, 1 Dec 2019
- When to Use Which User-Experience Research Methods — Nielsen Norman Group, 17 Jul 2022
- Card Sorting: Uncover Users' Mental Models for Better Information Architecture — Nielsen Norman Group, 2 Feb 2024
- A/B Testing 101 — Nielsen Norman Group, 30 Aug 2024
- User Journeys vs. User Flows — Nielsen Norman Group, 16 Apr 2023
- The Definition of User Experience (UX) — Nielsen Norman Group (Norman and Nielsen), 8 Aug 1998
- Fitts's Law and Its Applications in UX — Nielsen Norman Group, 31 Jul 2022
