3/19/2026 • 5 min read
Subject line A/B testing guide
How to test subject lines in a way that actually teaches you something.
Subject line tests fail when too many variables move at once.
What to isolate
Only test one major difference at a time:
- Specificity vs curiosity
- Personalized vs broad
- Short vs medium length
- Deadline cue vs no deadline cue
Put this advice into practice
Test your next subject line before you send it.
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