CRO Glossary
Flicker Effect
A brief flash of the original page content before a client-side A/B test variant renders and replaces it.
The flicker effect (sometimes called the FOOC — flash of original content) happens when a testing tool changes a page after it has already started rendering. The visitor sees the control version for a split second, then the page visibly shifts to the variant, which can look like a glitch or feel jarring.
This matters because it undermines the integrity of the experiment as well as the user experience. If visitors notice the swap, it can bias behavior (they may distrust the page or simply be distracted), and it can also make the control look artificially better since the variant effectively had a delayed, worse first impression. On slower connections or heavier scripts, flicker gets worse, which means your test results can be confounded with page speed rather than the actual change you're testing.
The usual causes are client-side (JavaScript-based) testing tools that wait for the page to load before injecting changes. Common fixes include anti-flicker snippets that hide the page until the variant is ready, moving logic higher in the page load, or switching to server-side testing where the variant is decided before the HTML is sent to the browser.
For example, a pricing page test that swaps a hero image via JavaScript might flash the old image for 200-400ms before the new one appears — enough for analytics to record a slightly different bounce pattern between control and variant, muddying the read on which version actually performs better.
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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