Define Phase
Purpose
The Define phase locks down what problem is being solved, for whom, and how success will be measured. Nothing later in DMAIC is safe until the charter is honest.
Core tools
Project charter. Contains the business case, problem statement, goal statement, scope (in and out), team, sponsor, and high-level milestones. A sound problem statement answers what, where, when, and how much - with data, no assigned cause, and no proposed solution. A sound goal statement is SMART: "Reduce Line 3 leaks from 4.2% to below 1.0% by 30 September."
VOC to CTQ. Voice of the Customer is raw, fuzzy customer language; Critical to Quality requirements translate it into measurable specifications. A VOC of "the phone charges too slowly" might become a CTQ of "full charge in under 90 minutes."
SIPOC. A one-page, high-level view: Suppliers, Inputs, Process (5–7 steps), Outputs, Customers. It is built in Define to set boundaries, not to document detailed process steps - a detailed flowchart belongs in Measure.
Kano model. Must-be (dissatisfiers), performance (more is better), and delighters (unexpected). A feature customers notice only when it is missing is a must-be.
Traps
Scope creep, charters that already name the root cause, goal statements with no baseline, and confusing VOC with CTQ are all classic distractors. Also confirm who the customer is - internal customers count too.
Worked example
Bad: "Packaging leaks because the sealer is old." Good: "From January to March, 4.2% of packages sealed on Line 3 leaked within 24 hours, against a customer requirement of 1.0%."