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Decision time grows logarithmically with the number of equally likely choices, so the tenth option costs far less than the second.

LAW·001Law · Decision making

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·002Law · Motor control

People spend most of their time on other products, so they expect yours to work the way those do.

LAW·003Law · 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·004Law · Memory

Every system has irreducible complexity. The only question is whether the user absorbs it or the product does.

LAW·005Law · Complexity

Below roughly 400 milliseconds of system response, people stop waiting on the machine and productivity rises sharply.

LAW·006Law · Performance

People judge an experience by its most intense moment and its ending, not by the average of the whole thing.

LAW·007Law · Memory

Items at the start and end of a list are remembered best, which makes the middle of any list the weakest position.

LAW·008Law · Memory

The item that differs from its neighbours is the one that gets noticed and remembered.

LAW·009Law · Attention

Unfinished tasks stay mentally active, which is why visible progress pulls people back to complete them.

LAW·010Law · Motivation

People perceive attractive interfaces as easier to use, and forgive small problems in them more readily.

LAW·011Law · 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·012Law · Robustness

Work expands to fill the time available, which is why open-ended tasks in an interface tend to sprawl.

LAW·013Law · Behaviour

Effort increases as a visible goal gets nearer, and artificial early progress accelerates the whole journey.

LAW·014Law · Motivation

Time to complete a task falls as a power function of repetitions, so expert speed depends on the interface staying stable.

LAW·015Law · 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·016Law · Motor control

The eye resolves complex arrangements into the simplest, most stable interpretation available.

LAW·017Law · Perception

Among competing solutions that meet the requirement, prefer the one with fewest assumptions and moving parts.

PRIN·001Principle · 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.

PRIN·002Principle · Prioritisation

The interface should always tell the user what is going on, through timely and appropriate feedback.

HEUR·001Heuristic · Nielsen heuristics

Use the words, concepts and order that your users already have, not the ones your database has.

HEUR·002Heuristic · Nielsen heuristics

Users make mistakes and change their minds, so every state needs a clearly marked way out.

HEUR·003Heuristic · Nielsen heuristics

The same thing should look and behave the same way everywhere, and follow platform convention where one exists.

HEUR·004Heuristic · Nielsen heuristics

Better than a good error message is a design in which the error cannot occur.

HEUR·005Heuristic · Nielsen heuristics

Recognising something on screen is far easier than remembering it, so keep the information visible.

HEUR·006Heuristic · Nielsen heuristics

Accelerators that experts use and novices ignore let one interface serve both populations.

HEUR·007Heuristic · Nielsen heuristics

Every extra element competes with the ones that matter, so content should earn its place on the screen.

HEUR·008Heuristic · Nielsen heuristics

Error messages should say what happened, why, and what to do next, in plain language.

HEUR·009Heuristic · Nielsen heuristics

Aim for a product that needs no explanation, then write help for the cases where that is impossible.

HEUR·010Heuristic · Nielsen heuristics

Show the common case first and reveal the rest on request, so complexity is available without being present.

PRIN·003Principle · Complexity

Every action needs a perceivable response that says it was received and what it did.

PRIN·004Principle · Interaction

A capability that users cannot find does not exist, no matter how well it is built.

PRIN·005Principle · Interaction

An interface people can explore without fear produces confident users and fewer support tickets.

PRIN·006Principle · Interaction

An affordance is what an object makes possible. A signifier is the cue that tells you so.

PRIN·007Principle · Interaction

Controls should be arranged to match the thing they control, so the relationship needs no explanation.

PRIN·008Principle · Interaction

Restricting what is possible is the cheapest way to prevent an entire class of error.

PRIN·009Principle · Interaction

Two gaps break interfaces: not knowing how to do the thing, and not knowing whether it worked.

PRIN·010Principle · Interaction

Working memory is a narrow channel, and interfaces spend it on three kinds of work, only one of which is useful.

PSY·001Psychology · Attention and memory

More options can reduce satisfaction and completion, but the effect is much less reliable than the popular version suggests.

PSY·002Psychology · Decision making

Users act on their belief about how the system works, not on how it actually works.

MOD·001Mental model · Understanding

People learn to ignore anything that looks like an advertisement, including your own content when it looks like one.

PSY·003Psychology · Attention

Focused attention makes people miss things in plain sight, including changes to the interface they are using.

PSY·004Psychology · Attention

Absorbed, effortless concentration occurs when challenge and skill are balanced, and interruptions destroy it.

PSY·005Psychology · Engagement

The first number a person sees shapes every judgement that follows, even when it is obviously arbitrary.

BIAS·001Cognitive bias · Judgement

Losing something feels worse than gaining the same thing feels good, which shapes how people react to change.

BIAS·002Cognitive bias · Judgement

The same fact produces different decisions depending on whether it is expressed as a gain or a loss.

BIAS·003Cognitive bias · Judgement

Most people accept whatever is preselected, which makes defaults the most powerful control in an interface.

BIAS·004Cognitive bias · Judgement

People prefer things to stay as they are, which is why every redesign starts with a deficit.

BIAS·005Cognitive bias · Judgement