During a new smartphone development project, after completing the design and initial development phases, customers are given a functional prototype for user acceptance testing. Multiple customers report critical issues: the phone constantly reboots during normal usage, the touch screen becomes unresponsive intermittently, and battery life is significantly below specifications. The project team needs to systematically identify all potential root causes of these defects to develop comprehensive solutions before the production phase. What tool should the project team use first to analyze and categorize the potential causes of these quality issues?