According to Carol Righi, PhD, user-centered design can be understood to include both art and science. Science exists in gathering market intelligence, user characteristics, wants and needs, tasks and feedback. But there is art in the process of creating an elegant design and in the different solutions different developers envision.
So how does the process move from science to art? Often, the team comes up with a 'subsumer,' a high-level entity that incorporates all the constituent requirements.
Things that facilitate design: