Decision time grows logarithmically with the number of equally likely choices, so the tenth option costs far less than the second.
Law · Decision making
Laws, principles, heuristics and the psychology underneath them
UX
The reasoning layer. Why an interface works, what the evidence actually says, and where the popular version of a law overstates its source.
64 entries across 24 categories
Laws17Cognitive biases14Principles11Heuristics10Psychology10Mental models2
Type
Category
Level
64 entries
Time to hit a target depends on how far away it is and how big it is, which makes size and distance the two levers you control.
Law · Motor control
People spend most of their time on other products, so they expect yours to work the way those do.
Law · Conventions
Working memory holds a small number of chunks, so the design lever is what counts as a chunk, not the raw item count.
Law · Memory
Every system has irreducible complexity. The only question is whether the user absorbs it or the product does.
Law · Complexity
Below roughly 400 milliseconds of system response, people stop waiting on the machine and productivity rises sharply.
Law · Performance
People judge an experience by its most intense moment and its ending, not by the average of the whole thing.
Law · Memory
Items at the start and end of a list are remembered best, which makes the middle of any list the weakest position.
Law · Memory
The item that differs from its neighbours is the one that gets noticed and remembered.
Law · Attention
Unfinished tasks stay mentally active, which is why visible progress pulls people back to complete them.
Law · Motivation
People perceive attractive interfaces as easier to use, and forgive small problems in them more readily.
Law · Perception
Be liberal in what you accept and conservative in what you send, which for interfaces means accepting messy input and returning clean output.
Law · Robustness
Work expands to fill the time available, which is why open-ended tasks in an interface tend to sprawl.
Law · Behaviour
Effort increases as a visible goal gets nearer, and artificial early progress accelerates the whole journey.
Law · Motivation
Time to complete a task falls as a power function of repetitions, so expert speed depends on the interface staying stable.
Law · Learning
Moving a pointer along a constrained path takes time proportional to the path length divided by its width, which is why nested menus are hard.
Law · Motor control
The eye resolves complex arrangements into the simplest, most stable interpretation available.
Law · Perception
Among competing solutions that meet the requirement, prefer the one with fewest assumptions and moving parts.
Principle · Complexity
A small share of features carries most of the use, which is a prompt to go and measure rather than an excuse to cut.
Principle · Prioritisation
The interface should always tell the user what is going on, through timely and appropriate feedback.
Heuristic · Nielsen heuristics
Use the words, concepts and order that your users already have, not the ones your database has.
Heuristic · Nielsen heuristics
Users make mistakes and change their minds, so every state needs a clearly marked way out.
Heuristic · Nielsen heuristics
The same thing should look and behave the same way everywhere, and follow platform convention where one exists.
Heuristic · Nielsen heuristics
Better than a good error message is a design in which the error cannot occur.
Heuristic · Nielsen heuristics
Recognising something on screen is far easier than remembering it, so keep the information visible.
Heuristic · Nielsen heuristics
Accelerators that experts use and novices ignore let one interface serve both populations.
Heuristic · Nielsen heuristics
Every extra element competes with the ones that matter, so content should earn its place on the screen.
Heuristic · Nielsen heuristics
Error messages should say what happened, why, and what to do next, in plain language.
Heuristic · Nielsen heuristics
Aim for a product that needs no explanation, then write help for the cases where that is impossible.
Heuristic · Nielsen heuristics
Show the common case first and reveal the rest on request, so complexity is available without being present.
Principle · Complexity
Every action needs a perceivable response that says it was received and what it did.
Principle · Interaction
A capability that users cannot find does not exist, no matter how well it is built.
Principle · Interaction
An interface people can explore without fear produces confident users and fewer support tickets.
Principle · Interaction
An affordance is what an object makes possible. A signifier is the cue that tells you so.
Principle · Interaction
Controls should be arranged to match the thing they control, so the relationship needs no explanation.
Principle · Interaction
Restricting what is possible is the cheapest way to prevent an entire class of error.
Principle · Interaction
Two gaps break interfaces: not knowing how to do the thing, and not knowing whether it worked.
Principle · Interaction
Working memory is a narrow channel, and interfaces spend it on three kinds of work, only one of which is useful.
Psychology · Attention and memory
More options can reduce satisfaction and completion, but the effect is much less reliable than the popular version suggests.
Psychology · Decision making
Users act on their belief about how the system works, not on how it actually works.
Mental model · Understanding
People learn to ignore anything that looks like an advertisement, including your own content when it looks like one.
Psychology · Attention
Focused attention makes people miss things in plain sight, including changes to the interface they are using.
Psychology · Attention
Absorbed, effortless concentration occurs when challenge and skill are balanced, and interruptions destroy it.
Psychology · Engagement
The first number a person sees shapes every judgement that follows, even when it is obviously arbitrary.
Cognitive bias · Judgement
Losing something feels worse than gaining the same thing feels good, which shapes how people react to change.
Cognitive bias · Judgement
The same fact produces different decisions depending on whether it is expressed as a gain or a loss.
Cognitive bias · Judgement
Most people accept whatever is preselected, which makes defaults the most powerful control in an interface.
Cognitive bias · Judgement
People prefer things to stay as they are, which is why every redesign starts with a deficit.
Cognitive bias · Judgement
People look to others' behaviour when deciding, especially under uncertainty.
Psychology · Influence
People give disproportionate weight to a source that appears authoritative, including visual markers of authority.
Cognitive bias · Influence
People value things more once they own them, and more still once they have put work into them.
Cognitive bias · Judgement
Once you know how something works you cannot accurately imagine not knowing, which is why teams misjudge their own interfaces.
Cognitive bias · Judgement
People seek and weight evidence that supports what they already believe, which quietly corrupts research findings.
Cognitive bias · Research integrity
Analysing only the people who made it through hides everything the failures would have told you.
Cognitive bias · Research integrity
People judge how likely something is by how easily an example comes to mind, which favours the recent and the vivid.
Cognitive bias · Judgement
Adding a deliberately inferior third option changes which of the original two people choose.
Cognitive bias · Judgement
A strong impression in one area spills over into unrelated judgements about the same thing.
Cognitive bias · Judgement
Users start acting immediately rather than reading, even when reading would be faster, so design has to teach through doing.
Psychology · Learning
Interfaces engineered to make users act against their own interest, now regulated in several jurisdictions.
Principle · Ethics
Learning spread over time is retained far better than the same amount of learning crammed into one session.
Psychology · Learning
Repeated exposure to something increases preference for it, which is why redesigns feel wrong before they feel right.
Psychology · Perception
Past investment that cannot be recovered still drives decisions, in users and in product teams alike.
Cognitive bias · Judgement
The claim that decision quality degrades over a sequence of choices. Popular, intuitive, and on much shakier experimental ground than its usage suggests.
Psychology · Attention and effort
Judge a design by the outcome it produces for a person and for the business, not by whether the request was delivered.
Mental model · Product
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