Data can help inform design choices at every step in the process.
Jen Matson is a user experience designer with more than 20 years of experience designing and building websites for companies such as Amazon, Comedy Central, LEGO, and T-Mobile. Currently, she is a Senior Manager of UX Design at Nordstrom.
Jen can be found on Twitter as @nstop.
It's important to know the "why" in addition to the "what" in UX design.