Sample Size Heuristics: How Many Users Are Enough?

One of the most frequently cited principles in UX research is Jakob Nielsen's discovery that 5 users uncover approximately 85% of usability defects in a qualitative test.

Why does this hold?

• The first participant uncovers ~31% of usability problems.
• The second participant repeats much of what the first said, adding novel insights.
• By the fifth participant, you enter diminishing returns where observers see the same core friction points repeated.