Experiment lifecycle and stopping¶
An A/B experiment prepares both versions, runs the selected shopper sessions, then finalizes the available evidence. You can follow progress while it runs or stop it yourself.
The phases of a run¶
When you click Run experiment, the run starts and moves through these phases on the experiment page.
- Warming up. Squoosh reserves the synthetic shoppers and prepares both versions of your page — Control page (A) and Variant page (B). No shoppers are on the floor yet. While the run is in this phase, the stop control reads Cancel launch.
- Shoppers entering. The first synthetic shoppers begin to shop both versions, and the floor starts to populate. The stop control changes to Stop.
- Live. The full run is underway. The shopper floor fills in as more shoppers finish, alongside a combined funnel and what individual shoppers did and said. The status reads Live.
- Completed. The shopper sessions have finished or otherwise ended, and Squoosh finalizes the available evidence. Completion does not guarantee enough usable evidence for a winner.
What you see while a run is Live¶
While a run is Live, the page shows the shopper floor, the combined funnel across both versions, and feedback from individual shoppers. It does not show a winner, a lift, or a confidence figure.
Those numbers are a finish-line reveal. They appear only once the run completes, so a partial result can't mislead you mid-run. For how to read them after the run, see How Squoosh decides a winner and Reading lift and confidence.
How long a run takes¶
A/B experiments run shoppers in parallel, up to 20 active sessions per experiment, as shared capacity allows. Each selected shopper runs on both versions: 15, 50 or 100 paired shoppers schedule 30, 100 or 200 sessions. Each free slot can admit the next session.
The launch footer shows Execution: Priority instead of a fixed time estimate. Duration depends on the sample size, site behavior and available capacity. Results arrive as shoppers finish.
Automatic completion¶
A/B experiments do not currently stop when an interim confidence score crosses a threshold. The selected sample runs to completion or another terminal condition before the result is finalized. Live progress is descriptive; it is not an early winner call.
This also applies to the Village conversion lift result that a 100-shopper run in Two URLs mode produces. It uses the sample and evidence requirements set at launch: a paired shopper counts only when a valid outcome was observed on both versions, a pair that fails is retried once, and missing pairs make the result insufficient rather than a guess. Auto-stop is always off for these runs. See Conversion-evidence results.
Conversion Reports have their own panel and completion rules. See Run a Conversion Report.
How a run ends¶
Every run reaches one of these final states. The state shows as the status on the experiment page and in the experiments list.
| Status | What it means | What to do |
|---|---|---|
| Completed | The run finished and Squoosh filed a verdict. | Read the verdict: ship the variant, keep the original, or re-test if it was too close. |
| Undecided | The run finished, but the two versions never separated enough to call a winner. | Re-test with a sharper change between A and B. This is not a failure. |
| Terminated | You stopped the run before it finished. | Launch a new experiment when you're ready. |
A run you haven't launched yet shows as Draft.
Stopping a run yourself¶
You can end a run early from the top of the experiment page:
- Before any shoppers are live, the control reads Cancel launch.
- Once the run is Live, it reads Stop.
Either way, the run ends in the Terminated state. It does not file a verdict.
Note
Squoosh attempts to recover stalled runs. An ended session may have no usable conversion evidence; read the final coverage and any failure reason rather than treating every ended session as a successful observation.