How important is A/B Testing?

Split your list!

A popular option for splitting your list is the 10/10/80 split. Use 10 percent of your list to test one option and 10 percent to test the other. The remaining 80 percent receives the best-performing message. Dividing your list randomly can help with getting an accurate cross-section of your overall recipients.

https://www.marketingcloud.com/blog/ab-testing-how-do-you-know-that-your-results-are-statistically-valid/