What makes a good subject line test?

A good subject line test follows solid experimental principles:

Clear hypothesis explaining what you expect and why. "Adding the recipient's name will increase opens because it signals personalized content."

Single variable change isolating one element difference. Test personalization OR length OR emoji, not all three simultaneously.

Meaningful contrast between versions. Testing "Get 20% off" versus "Get 20% off today" is too subtle. Test "Get 20% off" versus "Your exclusive discount inside."

Adequate sample size calculated before starting, typically several thousand per variant minimum.

Full-funnel measurement tracking not just opens but clicks and conversions. A subject line that boosts opens but tanks conversions is not a winner.

The best subject line tests teach you about your audience, not just which words work. Design tests that reveal preferences you can apply broadly.