Control
The Control domain sustains the gains and transfers ownership to the process owner. ICGB tests whether you build a control system that detects drift and triggers action.
Control plan
For each metric, define the target, measurement method, frequency, sample size, owner, and reaction plan. Add standard work, visual management, layered audits, and training records.
SPC
- Control limits come from the process, typically the mean ±3 sigma; specification limits come from the customer.
- Use Xbar-R or Xbar-s for subgroup data, I-MR for individual measurements, and p, np, c, or u charts for attributes.
- Out-of-control signals include a point beyond a limit, seven or eight points in a row on one side, a trend, and two of three points beyond two sigma.
Worked example
An Xbar chart shows one subgroup mean above the upper control limit. The team does not recalculate the limits. They check the reaction plan: new operator, material lot change, or measurement error. They find a new untrained operator on that shift. After retraining and removing the special-cause point, the process is stable again. Reasoning: control limits describe normal variation; a point beyond them signals a special cause that must be investigated, not hidden by recalculating limits.
Traps
- Treating control limits as specification limits.
- Recalculating limits to eliminate inconvenient points.
- A control chart with no reaction plan or named owner.
- Adjusting a stable process, which adds variation.
- Ending monitoring immediately after project closure.