5.0 Control

Lean Six Sigma Refresher: The Control Phase

The Control Phase ensures that improvements achieved in the Improve Phase are sustained over time. It’s about embedding solutions into daily operations, monitoring performance, and preventing regression. For certified practitioners, this phase is a discipline of consistency, accountability, and vigilance.

5.1 Lean Controls

5.1.1 Control Methods for 5S

  • Sort, Set in Order, Shine, Standardize, Sustain.

  • Visual workplace organization reduces waste and supports process stability.

  • Control methods include audits, checklists, and visual cues to maintain discipline.

5.1.2 Kanban

  • Pull-based scheduling system that signals demand.

  • Prevents overproduction and aligns workflow with customer needs.

  • Visual boards or cards ensure transparency and responsiveness.

5.1.3 Poka-Yoke (Mistake Proofing)

  • Simple devices or methods that prevent errors before they occur.

  • Examples: color coding, sensors, interlocks.

  • Reinforces quality at the source.

5.2 Statistical Process Control (SPC)

5.2.1 Data Collection for SPC

  • Reliable data is the foundation of control charts.

  • Define sampling frequency, subgroup size, and measurement methods.

5.2.2 I-MR Chart

  • Monitors individual values and moving ranges.

  • Useful for processes with low-volume or continuous data.

5.2.3 Xbar-R Chart

  • Tracks subgroup averages and ranges.

  • Ideal for small subgroup sizes (n ≤ 10).

5.2.4 U Chart

  • Monitors defects per unit.

  • Suitable for variable sample sizes.

5.2.5 P Chart

  • Tracks proportion of defective units.

  • Useful for attribute data with varying sample sizes.

5.2.6 NP Chart

  • Monitors number of defectives in constant sample sizes.

5.2.7 Xbar-S Chart

  • Uses subgroup averages and standard deviations.

  • Suitable for larger subgroup sizes (n > 10).

5.2.8 CuSum Chart

  • Cumulative sum chart detects small shifts quickly.

  • Effective for processes requiring tight control.

5.2.9 EWMA Chart

  • Exponentially Weighted Moving Average chart smooths data trends.

  • Sensitive to gradual shifts.

5.2.10 Control Methods

  • Establish rules for detecting out-of-control conditions.

  • Examples: points beyond limits, runs, trends.

5.2.11 Control Chart Anatomy

  • Key elements: center line, control limits, data points.

  • Visual representation of process stability.

5.2.12 Subgroups, Impact of Variation, Frequency of Sampling

  • Subgrouping isolates short-term variation.

  • Sampling frequency balances sensitivity with resource efficiency.

5.2.13 Center Line & Control Limit Calculations

  • Center line = process average.

  • Control limits = ±3 standard deviations from mean.

  • Calculations ensure statistical validity.

5.3 Six Sigma Control Plans

5.3.1 Cost Benefit Analysis

  • Evaluates whether controls justify their cost.

  • Ensures sustainability without unnecessary overhead.

5.3.2 Elements of the Control Plan

  • Defines monitoring methods, responsibilities, and corrective actions.

  • Documents how improvements will be maintained.

5.3.3 Elements of the Response Plan

  • Specifies actions when performance deviates.

  • Includes escalation paths and contingency measures.

Final Thoughts

The Control Phase is the safeguard of Lean Six Sigma. By embedding Lean controls, applying SPC, and formalizing control plans, practitioners ensure that gains are not temporary but become part of the organization’s DNA. This phase transforms improvements into lasting operational excellence.