I85PREP
⌘ K
CSCA study materials
CSCA study resources

CSCA: Lean and Six Sigma

CSCA · Work through your exam, one topic at a time.

All guide sections

Lean and Six Sigma

Lean and Six Sigma tests improvement method: Lean removes waste and builds flow, Six Sigma reduces variation and defects, and DMAIC is the shared project discipline.

Lean fundamentals

  • Value is defined by the customer; everything else is waste. The eight wastes are transport, inventory, motion, waiting, overproduction, overprocessing, defects and unused talent.
  • Takt time = available working time ÷ customer demand. It sets the heartbeat of the line; cycle time above takt means the line cannot meet demand without overtime, a second resource or kaizen.
  • Tools: value stream mapping, 5S, standard work, SMED for changeover, kanban pull, heijunka levelling, jidoka (stop the line), andon, poka-yoke.

Six Sigma toolkit

  • DMAIC: Define, Measure, Analyse, Improve, Control. DMADV/DFSS is for new designs.
  • Sigma level maps to DPMO; six sigma is 3.4 DPMO under the 1.5σ shift convention.
  • Capability: Cp = (USL − LSL)/6σ measures spread only; Cpk = min[(USL − μ)/3σ, (μ − LSL)/3σ] also penalises off-centre running. 1.33 is the usual minimum, 1.67 for critical characteristics.
  • MSA before capability, and a stable process before either. SPC charts separate common from special cause; control limits come from the process, specification limits come from the customer.
  • FMEA prioritises failure modes by RPN = severity × occurrence × detection.

Worked example

A characteristic has LSL 4.0, USL 10.0, mean 7.5 and σ 0.8. Cpk = min[(10 − 7.5)/2.4, (7.5 − 4.0)/2.4] = min[1.04, 1.46] = 1.04 - below 1.33, so the process is not capable. Centring the mean at 7.0 raises Cpk only to 1.25, still short. To reach 1.33 at a mean of 7.0, σ must fall to about 0.75 (3.0 ÷ (3 × 1.33)). Both centring and variation reduction are needed, and the arithmetic shows which is binding.

Traps

  • Reporting Cp and ignoring Cpk, or comparing control limits with specification limits.
  • Computing capability on an unstable process or before MSA.
  • Treating Lean as headcount reduction rather than waste removal.
  • Logging RPN scores without assigning owners and due dates.