It’s one of the most repeated rules of testing in email marketing; only test one criterion at a time. If you test two, the results are no longer statistically clear and precise, which is the what makes testing such an essential tool. Do it properly and you know. Anything else is a guess and will hit you where it hurts.

Once you have even a little experience, when you want to improve a metric you will have an idea as to what requires changing. For instance, with a low open rate, or one you feel needs elevating, the most obvious place to start is the Subject Line. All that work you put into the email marketing campaign is wasted if your subscribers fail to open the thing. 

The problem with this idea is that another truth of email marketing is that nothing works in isolation. The temptation might be to up the tone of the Subject Line, making it a lot bouncier and fun, but to do so requires a change in the body of the email as you don’t want to make you subscribers think it is going to be entertaining and then return to sober design.

In order to facilitate testing for a change in individual aspects of your email marketing campaign, almost every facet of it to be changed: images, tone of copy, landing page to reflect the new design and probably more.

If you want to change the tone of the email, perhaps by rummaging through the supply of free email marketing templates, you need to ensure the theme is continuoius. The returns from your test still stands as much as if you’ve only changed one thing because, in effect, you have: the tone. The data you receive will provide you with the answers you need.

If it divides that specific segmented email marketing list, with, say, 25% showing an improvement, the answer is not to simply implement the change of tone, but to work out why that quarter responded better. It might be age, outlook, or whatever. Once you’ve discovered the particular metric, you need to change the way you segment your email marketing lists.