When designing for the web, you can analyze usage data for your product and compare different interfaces in A/B tests. This is sometimes called “data-driven design”, but I prefer to think of it as data-informed design — the designer is still driving, not the data.
To make this work in practice it’s important to use the right metrics. Basic traffic metrics (like overall page views or number of unique users) are easy to track and give a good baseline on how your site is doing, but they are often not very useful for evaluating the impact of UX changes. This is because they are very general, and usually don’t relate directly to either the quality of the user experience or the goals of your project — it’s hard to make them actionable.