Defining what success looks like before you start any test is important because it helps you know when an assumption has been validated. Success should be clear and measurable. For example, if you're testing a pricing assumption, success might mean that 50% of users are willing to pay for a new premium subscription plan.

To define success, ask yourself: what outcome would prove this assumption right? Set clear goals, like how many users need to subscribe or how much they are willing to pay. Make sure to define success before you run the test. This way, you know exactly what to look for in the results and can make better decisions based on data.