Novelty effect: New designs perform better initially due to curiosity (fades in 1-2 weeks). Primacy effect: Original designs perform better because users are habituated (also fades in 1-2 weeks). Solution: Run tests for 3-4 weeks minimum and segment new vs returning users to detect these biases.
The Two Effects
Novelty Effect
New design performs better initially because users are curious
Primacy Effect
Original design performs better because users are used to it
Real Example
Scenario: You redesign your homepage with a bold new layout.
If you stopped after week 1: You'd think you had a 15% winner. Reality: 1-2% improvement.
How to Detect These Effects
Performance changes over time
Variant wins week 1, loses week 3
Different results for new vs returning
New users love it, returning users hate it
Engagement drops after initial spike
High initial engagement that fades
Prevention Strategies
Minimum Test Duration
To account for novelty and primacy effects:
- • Minimum 2 weeks for any test
- • 3-4 weeks for major redesigns (high novelty potential)
- • Segment by new vs returning to see if effects differ
- • Watch week-over-week trends for stabilization
Run Valid Tests
ExperimentHQ tracks your results over time so you can spot novelty/primacy effects. Run tests long enough to get true results.