CRO Glossary
Control Group
The unmodified experience shown to a portion of users, used as the baseline against which variants are measured.
The control group is the version of a page, flow, or feature that stays exactly as it currently exists in production. It represents the status quo and gives you a baseline to compare against any variant you're testing. Without a control, you have no way to know whether a change in conversion rate is due to your new idea or simply due to external factors like seasonality, a marketing campaign, or a site-wide outage.
In a typical A/B test, users are split via the randomization unit into a control and one or more treatment groups, with traffic allocation deciding what share sees each. The control isn't necessarily 'no design' — it's whatever currently exists, even if that design was itself the winner of a previous test.
A common mistake is letting the control group's experience quietly change during the test window (a bug fix, a content update) which contaminates the comparison. Keeping the control completely frozen for the test duration is essential for a clean read, and is one of the first things to check when investigating an unexpected result alongside things like SRM.
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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