A/B testing is a way to compare two versions of something to figure out which performs better. While it’s most often associated with websites and apps, the method is almost 100 years old and it’s one of the simplest forms of a randomized controlled experiment. This testing method has risen in popularity over the last couple of decades as companies have realized that the online environment is well-suited to help managers, especially marketers, answer questions like, “What is most likely to make people click? Or buy our product? Or register with our site?”. It’s now used to evaluate everything from website design to online offers to headlines to product descriptions. The test works by showing two sets of users (assigned at random when they visit the site) different versions of a product or site and then determining which influenced your success metric the most. While it’s an often-used method, there are several mistakes that managers make when doing A/B testing: reacting to early data without letting the test run its full course; looking at too many metrics instead of focusing on the ones they most care about; and not doing enough retesting to be sure they didn’t get false positive results.