CTQ and requirement trees

Learning objectives

• Construct a requirement tree from need to metric.
• Maintain logical linkage between branches.
• Keep solutions out of the tree.

Need → driver → CTQ

Begin with the need, branch to requirement drivers, then to measurable characteristics. Branches should collectively explain the need without unnecessary duplication.

Garment-factory example

"Durable seam" branches into seam strength, slippage, stitch security, appearance after laundering and defined test conditions.

Method

1. State the need at the root.
2. Ask "what does this require?" repeatedly.
3. Define measures at each leaf.
4. Trace every CTQ back to its VOC source.

Common mistakes

• Adding "buy new machine" as a requirement.
• Using overlapping branches.