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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 page
  1. Definition
  2. Why this matters for product decisions
  3. Designing for both ends of the curve
  4. In practice
  5. Sources
  6. Continue from here

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.

In practice

Command palettes in professional tools

Developer and design tools

The menu remains for discovery and the palette serves practice. Neither population is compromised, because the accelerator does not replace the discoverable path.

The cost of a navigation redesign

Enterprise software

Moving 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.

Sources

Where a source establishes something narrower than the popular reading of it, the note says so.

  • Mechanisms of skill acquisition and the law of practice

    A. Newell and P. S. Rosenbloom, in Cognitive Skills and Their Acquisition, 1981

  • 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

A Data TableAn Enterprise Workflow

Why this works

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Applied through

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Practical example

A worked decision where this was the deciding factor.

Related concept

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