What is a CTQ tree?
A CTQ tree (critical to quality tree) connects what customers say with what a process must deliver. Customer needs are usually broad and vague: "fast", "reliable", "easy". The tree asks, level by level, "what does that mean, and how would we measure it?" until each branch ends in a CTQ: a measurable characteristic with specification limits.
The result is a short list of metrics that together define success for the customer, and that become the focus of an improvement project or the requirements of a new design.
How to build a CTQ tree
- Start from the voice of the customer. Pick one important need from your VOC analysis.
- Find the drivers. Ask "what has to be true for this need to be met?" Aim for two to five drivers.
- Define CTQs. For each driver, ask "how would we measure it?" Write a metric with a unit.
- Set limits. Agree the specification limits and target with the customer, or derive them from what the customer accepts.
- Measure today. Compare current performance with each specification to see where the gaps are.
CTQ tree example
A contract manufacturer starts from its customers' top need: batches delivered when promised. Four drivers follow: production finishes on schedule, QA release is fast, materials are available, and the customer hears about delays early. Each driver gets one or two CTQs with limits, for example batch record review time with a USL of 5 days and schedule adherence with an LSL of 95%. Comparing the current values shows four CTQs out of specification: schedule adherence (88%), unplanned downtime (9 h per week), review time (7.5 days) and review queries (3.4 per record). Supplier on-time delivery meets its limit, and the delay notice is not measured yet. The out-of-spec CTQs become candidate DMAIC projects.
Common mistakes
- CTQs without limits. A metric without a specification cannot tell you whether the customer is satisfied.
- Solutions instead of requirements. "Install a new LIMS" is a solution; "review time ≤ 5 days" is the requirement.
- Internal limits. Specification limits should reflect what the customer accepts, not what the process does today.
- Too many CTQs. Focus on the few that drive the need; others can be monitored elsewhere.
- Check whether you can measure each CTQ reliably with a measurement system analysis.
How to use this CTQ Tree tool
- State the need. Click the first box and write the customer need, ideally from your voice of the customer data.
- Add the drivers. Click + next to the need to add the few things that must be true for it to be met.
- Add the CTQs. Click + next to each driver to add measurable CTQs.
- Set the specification. Click a CTQ to enter its unit, specification limits, target and how it is measured.
- Compare with today. Enter the current value of each CTQ to see whether it meets the specification.
- Export. Export the tree and the CTQ table to PDF, PNG or Excel.
Frequently asked questions
What is a CTQ tree?
A CTQ (critical to quality) tree breaks a broad customer need down into its drivers and then into specific, measurable requirements called CTQs. Each CTQ has a metric and specification limits, so a team knows exactly what to measure and what "good" means.
What is the difference between a need, a driver and a CTQ?
The need is what the customer wants, in their words ("batches delivered when promised"). Drivers are the few things that must be true to meet the need ("QA release is fast"). CTQs are measurable characteristics of each driver with limits ("batch record review time ≤ 5 days").
What makes a good CTQ?
A good CTQ is measurable, has an operational definition (what is measured, how and how often), has specification limits that reflect what the customer will accept, and is something the process can influence. "Fast release" is not a CTQ; "working days from record submitted to QA approval, USL 5" is.
What are LSL, USL and target?
The lower and upper specification limits are the smallest and largest values the customer accepts; the target is the ideal value. Many CTQs have only one limit: a time has an upper limit, a percentage on time has a lower limit.
Where does the CTQ tree fit in Six Sigma?
In DMAIC, the CTQ tree is a Define-phase tool that turns the voice of the customer into the project metric (the Y). In design for Six Sigma, CTQs become the design requirements that are predicted and verified on the design scorecard.
How many levels should a CTQ tree have?
Usually three: need, drivers, CTQs. Complex needs can have sub-drivers. Stop when every branch ends in something you can measure.