How do I measure the success of a subject line test (opens vs. clicks/conversions)?
Subject line tests require layered measurement:
Primary metric: Open rate since subject lines directly influence the open decision. This is your main comparison point for declaring winners.
Secondary validation: Click rate confirming that opens translate to engagement. A clickbait subject line may win opens but lose clicks if content disappoints.
Tertiary validation: Conversions ensuring the full funnel benefits. The ultimate goal is business outcomes, not vanity metrics.
Watch for misalignment where a subject line wins opens but loses downstream metrics. This usually indicates a disconnect between promise and delivery.
Consider engagement quality including time on page, pages visited, or purchase value for converted recipients.
Open rate wins that do not survive downstream validation are losses in disguise. Always verify that surface metrics translate to real outcomes.
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