How to use this tool
Enter visitors and conversions for version A and version B. The tool runs a two-sided z-test and tells you if the difference is significant.
What confidence level should I use?
95 percent is the common standard. Use 99 percent when the change is expensive or risky.
When should I stop a test?
Decide the sample size in advance and stop then. Stopping the moment the result looks significant inflates false positives.