Checking experiment results every day can make a random early gap look like a win. Sequential monitoring requires a prespecified rule for reading interim evidence and stopping.
In the demo, B's early lead disappears on later days, and the decision opens only at the planned endpoint. A fixed-horizon test should not be stopped opportunistically.
A planned sequential method is different from casual peeking. Record the statistical procedure, minimum useful effect, and guardrails before launch.
When to use
Use it when stakeholders want interim decisions; set the stopping rule first.