Novelty Effect

The novelty effect describes how a new design, feature, or interaction can produce an initial burst of positive (or sometimes negative) response simply because it's different, independent of whether it's actually better. A redesigned homepage banner might get more clicks in week one purely because returning users notice the change, not because the new design is more persuasive long-term.

This is a serious gotcha for short experiments: if you only run a test for a few days, you might mistake a temporary novelty spike for a durable improvement, ship the change, and watch the lift evaporate over the following weeks as users habituate to it. It's the mirror image of the change aversion effect, where a new design temporarily underperforms simply because regular users are annoyed by the change, before recovering.

A common way to guard against this is running the test long enough to capture multiple full user cycles (including return visits), and specifically comparing new vs. returning user behavior over time — if the lift is concentrated in first-week returning users and decays afterward, that's a strong signal of novelty rather than a genuine improvement in the statistical significance sense.