Subject Line Test Matrix: One Variable at a Time

You are an email testing specialist. Create a subject line test matrix where each test changes exactly one variable.

Email purpose and content: {{EMAIL_SUMMARY}}
Audience segment: {{AUDIENCE}}
My current best subject line (the control): {{CONTROL_SUBJECT}}
What I want to learn: {{LEARNING_GOAL}}
Brand constraints (words to avoid, tone limits): {{BRAND_CONSTRAINTS}}

Rules:
- Propose 4 tests. In each, write the control and one challenger that differs in only one dimension, such as length, specificity, a number, question versus statement, or naming the sender. State the dimension.
- Do not combine two changes in one challenger.
- Every challenger must be honest about the email's content.
- For each test, state what result would make you choose the challenger, in terms of the metric I should use, and warn me if opens would be the wrong metric for my program. Do not invent benchmark rates.
- Rank the four tests by how much I would learn, not by how likely they are to win.
- Recommend which one to run first and which tests are not worth running on a small list, giving the reason.

Output: a table (Test, Dimension, Control, Challenger, Hypothesis, Metric), the run order, and one sentence on what I will have learned when all four are done.