A/B testing email subject lines is one of the few marketing activities where you get real data about what works for your specific audience - not industry averages or general best practices, but evidence from your own list about how your subscribers actually behave. Done correctly, it compounds: each test teaches you something that makes the next test more precise. Done incorrectly, it produces noise. Small sample sizes, too many variables, poor baseline metrics, and confirmation bias all undermine the value of email A/B tests. This guide covers the correct method from setup through interpretation. Photo by Lukas Blazek on Pexels Why Subject Line A/B Tests Fail Most subject line A/B tests fail to produce actionable insights for one of three reasons. Sample size is too small. For statistical significance at 95% confidence, you need roughly 1,000 recipients per variant for a 5% detectable difference in open rates.…