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EXIN LSSA Six Sigma Green Belt study materials
EXIN LSSA Six Sigma Green Belt study resources

EXIN LSSA Six Sigma Green Belt: Improve and Control Phases

EXIN LSSA Six Sigma Green Belt · Work through your exam, one topic at a time.

All guide sections

Improve and Control Phases

Improve selects and tests solutions against verified causes; Control locks the gain into daily work. The exam tests solution selection, pilots, DOE basics, Lean countermeasures, control plans, SPC chart choice, and handover.

Core rules

  • Link every solution to a verified root cause. Prioritize by impact, effort, risk, and reversibility; update FMEA before implementation.
  • Pilot before full rollout with predefined success criteria and a comparison plan. For multiple factors, use DOE with randomization and replication rather than one-factor-at-a-time trials.
  • Use Lean countermeasures where flow or waste dominates: standard work, pull, 5S, quick changeover, poka-yoke, and visual management.
  • The control plan states what is monitored, who monitors it, frequency, specification or target, and the reaction when a signal occurs.
  • Choose SPC charts by data: Xbar-R for subgrouped continuous data, individuals charts for slow or single-stream processes, p or np for proportion defective, c or u for defect counts. Control limits come from a stable baseline, not from specifications.
  • Close with process-owner handover, documented standards, training, audits, and validated benefits.

Worked example

A p-chart baseline has p=0.04 and subgroup n=200. UCL = 0.04 + 3 x square root(0.04 x 0.96/200) = 0.04 + 3 x 0.01386 = 0.0816. LCL is below zero, so set it to zero. The next lot has 30 defective units out of 200 inspected units, or 0.15, above the UCL. This is a special-cause signal: investigate the lot, material, or operator before recalculating limits. Recalculating limits immediately would hide the signal and weaken the control system.

Traps that cost marks

  • Setting control limits at specification limits.
  • Recalculating limits after every out-of-control point to make the chart look good.
  • Writing a control plan with no reaction or owner.
  • Declaring a pilot successful without replication or a comparison baseline.
  • Running DOE without randomization, blocking, or replication.
  • Handing over without standard work and training.
  • Claiming sustained savings before the process owner accepts the control plan.