Time is a variable, and sending Test Email A in the morning and Test Email B in the afternoon can yield very different responses. So send tests out as near to the same time, same day as possible.

This does not mean that you should avoid testing different times of day and different days of the week. Quite the contrary, you should expect to test these variables. What’s important is keeping test emails together as you perform the tests.

You may even find different test results at different times of day. For example, Test Email A may do better than Test Email B on Monday afternoons, but on Wednesday morning, Test Email B could work better.

Next: Focus on statistical relevancy