The study files

Interaction laws

How people move, choose and wait.

• Fitts's Law: The farther away and the smaller a target is, the longer it takes to reach.
• Hick's Law: The more choices you show, the longer people take to decide.
• Jakob's Law: People spend most of their time on other products, so they expect yours to work the same way.
• Doherty Threshold: People stay engaged when a system responds in under about 400 milliseconds.
• Tesler's Law: Every system has some complexity that cannot be removed, so someone has to handle it.
• Postel's Law: Be flexible in what you accept from people, and careful and clear in what you give back.
• Pareto Principle: A large share of results often comes from a small share of causes, roughly 80 to 20.
• Parkinson's Law: Any task will expand to fill the time available for it.
• Occam's Razor: Among solutions that work equally well, prefer the simplest.

Memory and attention

What people can notice and hold in mind.

• Miller's Law: People can hold only a handful of items in short-term memory, famously 7 plus or minus 2.
• Chunking: Grouping items into meaningful units makes them easier to scan, understand and remember.
• Working Memory: The small, temporary mental space where people hold and use information during a task.
• Cognitive Load: The amount of mental effort a design asks for.
• Choice Overload: Too many options can make people feel overwhelmed, and sometimes choose nothing.
• Selective Attention: People focus on what relates to their goal, and filter out the rest.
• Serial Position Effect: People remember the first and last items in a series best.
• Von Restorff Effect: The item that looks different from the rest is the one people notice and remember.

Perception

How people see structure on a screen.

• Law of Proximity: Things placed close together are seen as a group.
• Law of Similarity: Things that look alike are seen as related, or as doing the same job.
• Law of Common Region: Things inside a shared boundary are seen as a group.
• Law of Uniform Connectedness: Things joined visually, such as by a line, are seen as more related than unconnected things.
• Law of Prägnanz: People see complex shapes in the simplest form they can.
• Aesthetic-Usability Effect: People tend to see attractive designs as easier to use.

Behavior

Why people act, continue and remember.

• Mental Models: The beliefs people hold about how a product works, based on their experience.
• Flow: The state of full focus and enjoyment that happens when a challenge matches your skill.
• Goal-Gradient Effect: People speed up as they get closer to a goal.
• Zeigarnik Effect: People remember unfinished tasks better than finished ones.
• Peak-End Rule: People judge an experience mostly by its most intense moment and its ending.
• Paradox of the Active User: People start using a product right away, without reading the instructions.
• Loss Aversion: A loss feels stronger to people than a gain of the same size.
• Framing Effect: The same facts can lead to different decisions, depending on how they are presented.
• Default Effect: People tend to keep whatever option is already selected.
• Scarcity Effect: People value something more when it seems limited or about to run out.
• Reactance: When people feel pushed or trapped, they resist, sometimes against their own interest.
• Temptation Bundling: Pairing something people want with something they are less keen on makes both more acceptable.
• Labor Illusion: People value and trust a result more when they can see the work behind it.
• Cognitive Bias: Mental shortcuts that make thinking faster, but can lead to systematic errors.

Heuristics

Checklists for judging any design.

• Nielsen's 10 Usability Heuristics: Ten broad rules of thumb for finding usability problems in any interface.
• Consistency and Standards: People should not have to wonder whether different words, situations or actions mean the same thing.
• Norman's Design Principles: Core ideas from Don Norman on how people understand and use things.
• Shneiderman's 8 Golden Rules: Eight principles for interface design from Ben Shneiderman.
• WCAG POUR Principles: The four principles behind web accessibility: Perceivable, Operable, Understandable and Robust.

Metrics

How to tell whether a design is working.

• Aha Moment: The point when someone first feels why a product is valuable to them.
• Selection Bias: A false conclusion that comes from comparing groups who were different from the start.
• Goodhart's Law: When a measure becomes a target, it stops being a good measure.
• Task Success Rate: The share of people who complete a task correctly.
• Time on Task: How long people take to finish a task.
• Error Rate: How often people make a mistake, per opportunity for an error.
• Completion Rate: The share of people who start a flow and finish it.
• Conversion Rate: The share of visitors who take the action you care about, such as buying.
• Funnel Drop-off: The share of people lost between one step of a journey and the next.
• Bounce Rate: The share of visits that end after one page, with no further action.
• Activation Rate: The share of new users who reach the first moment of real value.
• Retention Rate: The share of a group of users who are still active after some time.
• Churn Rate: The share of users or customers who leave during a period.
• Stickiness (DAU/MAU): How often monthly users come back on a typical day.
• System Usability Scale (SUS): A 10-question survey that gives a usability score from 0 to 100.
• Single Ease Question (SEQ): One question after a task: overall, how easy or difficult was it, on a 7-point scale?
• Net Promoter Score (NPS): A measure of how likely people are to recommend you, from −100 to +100.
• Customer Satisfaction (CSAT): The share of people who say they are satisfied with an experience.
• Customer Effort Score (CES): How much effort people felt they had to spend on something.
• HEART Framework: Google's way to choose UX metrics: Happiness, Engagement, Adoption, Retention and Task success.

Formulas

The math behind UX decisions, with calculators.

• Fitts's Law Formula: Predicts how long it takes to reach a target from its distance and size.
• Hick's Law Formula: Predicts decision time from the number of equally likely choices.
• SUS Scoring: Turns ten answers from 1 to 5 into a score from 0 to 100.
• NPS Formula: Subtracts the percentage of detractors from the percentage of promoters.
• WCAG Contrast Ratio: Measures how different text and background are in brightness, from 1:1 to 21:1.
• How Many Users to Test: Estimates the share of usability problems found as you add test users.
• Keystroke-Level Model (KLM): Predicts how long an expert takes on a task by adding up standard action times.
• Margin of Error: Shows how far a measured percentage could be from the true value.
• A/B Test Significance: Checks whether a difference between two versions is likely to be real, not chance.
• Modular Type Scale: Sets font sizes by multiplying a base size by a constant ratio.