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Statistical Power

Test duration depends on traffic, baseline rate, and minimum detectable effect.

How to apply it

Use a sample-size calculator before launching. Stopping early inflates false positives.

Practise it on a real challenge

CTA Copy — 'Get started' vs 'Try free for 14 days'

PM wants to test 'Get started' vs 'Try free for 14 days'.

More UX laws

  • 60-30-10 Rule
  • 8-pt Spacing Scale
  • Accessible contrast
  • Aesthetic-Usability Effect
  • Alignment Principle
  • Calibrated Trust
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