Flickering

Flickering is the brief visual flash or content shift that occurs when a page initially loads with original content and then visibly changes to display an A/B test variation after the testing script executes. It creates a jarring user experience where visitors see the page transform before their eyes.

Meaning & Context

Flickering happens because most client-side A/B testing tools operate asynchronously—the page begins rendering before the testing script determines which variation to show and applies the changes. This delay, often just milliseconds, is enough for users to perceive the original content before it switches. Flickering is most noticeable with above-the-fold changes and on slower connections.

Why It Matters

Flickering damages user experience, reduces trust, and can significantly bias test results by causing visitors to bounce or behave differently than they would with a smooth page load. It's particularly problematic for tests involving major visual changes and can make variation pages appear slower or lower quality than control. Minimizing flickering is essential for both valid results and maintaining brand perception.

Example

An e-commerce site testing a new navigation menu experiences flickering when visitors briefly see the old menu structure for half a second before it suddenly transforms into the new layout. This visual glitch confuses users and may cause them to click before the layout stabilizes, creating invalid interaction data.