CRO Glossary
Primacy Effect
A temporary underperformance of a new variant caused by existing users' familiarity with the old experience, the opposite of the novelty effect.
The primacy effect describes when returning users initially perform worse with a new design simply because it's unfamiliar and disrupts habits they've built around the old version, not because the new design is actually worse. Over time, as users adjust, the true effect of the change emerges, often more positively than the early data suggested.
This is essentially the mirror image of the novelty effect, where a new variant gets an artificial short-term boost from curiosity. Both effects distort early results and are reasons experimenters run tests long enough, and monitor results over time by user cohort (new vs. returning), rather than calling a winner after a few days.
A concrete example: redesigning primary navigation on a SaaS dashboard often shows a short-term dip in task completion or click-through rate among long-time users who know the old layout by muscle memory. If the team stops the test after three days and sees a negative result, they may kill a genuinely better design. Segmenting by new vs. returning users, or extending the test duration, helps distinguish a true regression from temporary friction caused by change itself.
Related terms
Above vs Below Median Split
A segmentation technique that divides users into two groups at the median of a metric to compare high and low engagers.
Qualitative Feedback Loop
A recurring process of collecting and acting on user comments, session recordings, and survey responses to inform test hypotheses.
Experiment Metadata
The structured record of who, what, when, and how for a test — hypothesis, owner, dates, variants, and metrics used to track and audit it.
Pre-Test Analysis
The upfront work of validating traffic, metrics, and baseline data quality before launching an experiment.
Above-the-Fold vs Below-the-Fold Messaging Hierarchy
How the order and prominence of value-proposition messaging is split between the visible screen and the rest of the page.
Experiment Contamination
When a control or variant group is unintentionally exposed to the other condition, corrupting the comparison.
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