Email subject line A/B test: framework that lifts open rate

Subject line determines 80% of email opens. Yet most SMEs launch 1 version and stop. A/B test framework = 20-40% more opens in 60 days.

What to test (in priority)

• Length (short vs long)
• Personalisation (name vs none)
• Question vs statement
• Numbers vs no numbers
• Emoji vs no emoji
• Tone (formal vs casual)
• Urgency vs no urgency

Technical setup

• 50/50 audience split
• Min sample 1,000 per variant
• Test for 4-6h (open rate stabilises after 4h)
• Winner: >15% difference for significance

Examples that won in real tests

• "3 SEO mistakes costing you clients" beat "March SEO newsletter" (+47%)
• "Marco, got 2 minutes?" beat "I'd like to ask you something" (+28%)
• "Last call: -20% ends at 11:59pm" beat "20% discount" (+33%)

What NOT to do

• Test only 100 contacts (sample too small)
• Change body content between variants (confused test)
• Test 4 things simultaneously (multi-var without volume)
• Premature test stop at first signal

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