How do I prioritize what to test?

You've got a list of things you could test: subject lines, send times, CTA copy, personalization, preview text, layout. You can't do them all at once. So how do you decide where to start?

The honest answer is a two-axis check. For every test you're considering, ask two questions: how big could the impact be, and how much work does it take to run? That gives you four buckets, and the order is pretty obvious from there.

High impact, low effort tests go first. Subject lines sit here. They affect every single open rate across every send, they're quick to write, and your ESP almost certainly has split testing built in. Send time testing also lives in this bucket. Easy to set up, and the results apply broadly.

High impact, high effort tests come next, once you've captured the easy wins. Things like a full CTA redesign, a new email template, or deep personalization using dynamic content. Worth doing, but save them for when you've already got the quick stuff dialed in.

Low impact tests (whatever the effort level) should sit at the bottom of your list unless you genuinely have spare bandwidth. Testing button color when your subject lines haven't been optimized is a great way to feel productive while not actually moving anything.

A simple scoring approach that works well in practice is the ICE model. Score each test idea on three things: Impact (how much could this improve performance?), Confidence (how sure are you it'll make a difference?), and Ease (how fast can you actually run it?). Rate each 1-10, average the scores, and sort the list. It's not scientific, but it stops you from spending six weeks testing something that affects 2% of your emails.

But one thing worth remembering: you want your tests to follow a clear hypothesis cycle. A test without a hypothesis is just noise. Before you run anything, write down what you expect to happen and why. That discipline also helps you document what you've learned, which matters a lot when you're six months in and can't remember why you changed your CTA wording.

Here's a rough order that works for most senders starting out:

1. Subject line testing (easy, broad impact on opens)
2. Send day and time (easy, affects engagement across the board)
3. CTA copy and placement (moderate effort, direct revenue impact)
4. Preview text and sender name (easy, often overlooked)
5. Personalization and segmentation changes (higher effort, higher ceiling)
6. Template and layout overhauls (high effort, do these last)

Once you're running tests consistently, the next step is making sure your learnings don't disappear. Documenting what you find is what separates a testing habit from a testing archive.

Not sure where your current setup even has room to improve? Our free SOS call is a good place to talk through what's worth testing first for your specific situation.