CRO Glossary
Server-Side Testing
Running A/B tests by changing logic on the backend rather than in the browser, allowing deeper and more complex variants.
Server-side testing means the variant assignment and rendering logic happen on the application server or in a service layer, rather than being injected into the page client-side with JavaScript after it loads. This is the opposite of typical client-side visual editor tools, which swap DOM elements in the browser once the page has already rendered.
The main advantage is that server-side tests can touch things a client-side script can't easily reach: pricing logic, algorithm changes, backend recommendation engines, checkout flow restructuring, or anything requiring a database query. It also avoids the visible 'flicker' of content changing after page load, which can itself bias results by making users notice they're in an experiment.
The tradeoff is speed of implementation: server-side tests typically require engineering time to build and ship behind a feature flag, whereas a marketer can often launch a simple client-side test alone. Many mature CRO teams run quick copy and layout tests client-side but move anything structural, like a new checkout flow or pricing test, to server-side implementation to reduce flicker and enable more powerful variants.
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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