Traffic Allocation

Traffic allocation determines how incoming users are split across the control and treatment groups in a test. A simple 50/50 split is the most common choice because it gets you to a reliable read as fast as possible, but teams sometimes use uneven splits — like 90/10 — to limit exposure to a risky new variant while still collecting data.

The allocation you choose directly affects how long a test needs to run to reach adequate statistical power. Shifting more traffic away from an even split increases the sample size required to detect the same effect, since the smaller group becomes the limiting factor. This is a key trade-off when a team wants to be cautious about a bold redesign but also wants results quickly.

For example, an e-commerce site testing a new checkout flow might allocate only 20% of traffic to the new flow for the first week to cap downside risk, then move to 50/50 once initial guardrail checks look healthy. Allocation decisions should be made before launch and generally not changed mid-test, since shifting proportions partway through can distort results and complicates analysis, and can even trigger an SRM-like alert if not tracked carefully.