Power Law of Practice
Time to complete a task falls as a power function of repetitions, so expert speed depends on the interface staying stable.
Definition
Newell and Rosenbloom (1981) showed that the time to perform a task decreases as a power function of the number of trials. Improvement is rapid at first and then flattens, approaching a floor set by the interface itself.
Established research. Originates in published research and has been replicated. The effect is real. Its size in your product still needs measuring.
On this plate
Reach for this when
- Designing tools used daily by the same people.
- Weighing a redesign against the cost to existing users.
- Deciding whether to add a confirmation step.
Common mistakes
- Optimising exclusively for first use.
- Removing accelerators during a visual redesign.
- Reading aggregate task time without segmenting by experience.
- Measuring a new interface on first-use performance only, which flatters designs that never get faster.
Why this matters for product decisions
It means novice performance and expert performance are different design targets, and optimising for one can damage the other. Wizards, tooltips and confirmation dialogs help on trial one and cost on trial one thousand.
It also means the floor matters. Practice reduces the time a user spends deciding, but it cannot reduce the time the interface itself imposes. If a frequent task requires four clicks, a heavily practised user still pays four clicks forever.
Designing for both ends of the curve
- Provide accelerators that sit alongside the discoverable path: keyboard shortcuts, command palettes, bulk actions, saved views.
- Keep layouts stable. Every change resets part of the learning curve, which is the real cost of a redesign.
- Let confirmation dialogs be dismissed permanently for users who have demonstrated competence, with an undo as the safety net.
- Measure task time by user tenure, not in aggregate. An aggregate median hides both populations.
One interface, two populations
The same interface serves someone on their first attempt and someone on their thousandth. These are the levers that serve both without forking the product.
| Need | Beginner | Practised user |
|---|---|---|
| Finding a command | Visible menu with labels | Palette or shortcut, taught by the menu |
| Repeating an action | One at a time, with confirmation | Bulk selection and repeat |
| Recovering | Explicit error messages | Undo, and no dialogue at all |
| Layout | Stable, predictable positions | Stable, predictable positions. This one does not change |
In practice
- i.
Command palettes in professional tools
Developer and design toolsThe menu remains for discovery and the palette serves practice. Neither population is compromised, because the accelerator does not replace the discoverable path.
- ii.
The cost of a navigation redesign
Enterprise softwareMoving a frequently used item resets practice for every existing user. That cost is real and quantifiable, and it belongs in the decision alongside the benefit to new users.
- iii.
Why the layout must not move
What practice buysPractice converts deliberate searching into motor memory tied to position. A redesign that relocates familiar controls does not cost a moment of confusion, it resets months of accumulated speed.
Local chart
LAW·015 · Region I · Grid A2
This entry and the entries it is stated to connect with. Each line is labelled with what the connection means.
Sources
Where a source establishes something narrower than the popular reading of it, the note says so.
- [1]
Mechanisms of skill acquisition and the law of practice
A. Newell and P. S. Rosenbloom, in Cognitive Skills and Their Acquisition, 1981
- [2]
The power law repealed: the case for an exponential law of practice
A. Heathcote, S. Brown and D. J. K. Mewhort, Psychonomic Bulletin and Review, 2000
Continue from here
Each link says what the connection is, so you can tell a principle from an alternative from a thing people mix this up with.
Applies when you are designing
Why this works
The reasoning this entry rests on.
- Status Quo BiasBIAS·005 · Cognitive bias
- FlowPSY·005 · Psychology
- Spacing EffectPSY·008 · Psychology
Applied through
Where this shows up as a concrete interface decision.
- Command PalettePAT·018 · Pattern
Practical example
A worked decision where this was the deciding factor.
- Choosing a Density for an Analytics DashboardCASE·004 · Case study
Related concept
Connected closely enough to change how you apply this.
- Keyboard NavigationA11Y·002 · Accessibility
- Consistency and StandardsHEUR·004 · Heuristic
- Flexibility and Efficiency of UseHEUR·007 · Heuristic
- Recognition Over RecallHEUR·006 · Heuristic
- Fitts's LawLAW·002 · Law
- Steering LawLAW·016 · Law