Control Phase
Purpose and deliverables
Control locks in the gains. Deliverables: a control plan, updated standard work, monitoring (control charts), response plan, and project handoff/closure with documented lessons learned.
Core concepts
- Control plan: what is monitored, by whom, how often, with what reaction when out of spec.
- Control charts: choose by data type - X̄-R (continuous, subgroups), I-MR (continuous, individual), p (proportion defective), np (count defective), c (count of defects per unit), u (defects per unit with varying sample size).
- Control limits vs. specification limits: control limits come from the process (voice of the process); spec limits come from the customer (voice of the customer). Never plot spec limits as control limits.
- Western Electric / run rules: trends, runs, points beyond limits signal special cause.
- Sustainment: audits, dashboards, response plans, ownership transfer to the process owner.
- Project closure: financial validation, lessons learned, recognition.
Traps in questions
- Putting specification limits on a control chart as if they were control limits.
- Using a p-chart for defect counts rather than proportions, or c vs. u incorrectly (u handles varying sample size).
- Declaring victory without a response plan for out-of-control signals.
- Skipping handoff - gains erode when the process owner is not engaged.
- Forgetting to verify financial results and document lessons learned.
Rule of thumb
Control phase = monitor + respond + own. If no one is named to react when a signal appears, the control plan is incomplete.
Worked example
After reducing invoice errors, the team plots weekly p-charts (proportion defective). A point above the upper control limit triggers the documented response: the process owner audits the prior week's batches and retrains the responsible clerk. Spec limits (customer tolerance) stay on the capability report, not the chart.