~35% of "winning" UI changes lose half their lift within three weeks.

~35% of "winning" UI changes lose half their lift within three weeks.
That's the novelty effect — returning users react to change, not to the change being better. The curve decays toward the control.
— Segment new vs returning visitors: if the win lives only in returning users early on, suspect novelty.
— Hold the test one full purchase cycle before you trust the number.
First-week lift ▮▮▮▮▮▮ → week-three lift ▮▮▮.
Read the number, not the story. [new vs returning split · 21d window]