Why sample size gets calculated before a test launches
A test stopped as soon as one variant pulls ahead is one of the most common ways CRO produces wrong answers, since early leads in a test frequently reverse as more data comes in, a pattern well documented in how statistical significance behaves over time.
We calculate the required sample size from current traffic and baseline conversion rate before a test starts, set that as the stopping point in advance, and hold to it even when an early trend looks encouraging or discouraging. This is slower than calling a test after a few days, and it is also the difference between a result you can act on and one you cannot.








