Section VI: Control Phase 15 questions
Control carries 15 of the 100 scored questions on the 2022 CSSGB exam (up from 11 in the 2014 BoK). It covers statistical process control, the control plan, and the lean tools that keep a process in control: total productive maintenance (TPM) and the visual factory.
Key topics
| Topic | What to know |
|---|---|
| SPC basics | Common vs. special cause variation; measuring and monitoring process performance for continuous and discrete data. |
| Rational subgrouping | Form subgroups from units made under essentially the same conditions, so within-subgroup variation is common cause only. |
| Control charts | Selecting, building and interpreting I-MR, X̄-R, X̄-s, p, np, c and u charts; run rules for out-of-control signals. |
| Control plan | What is monitored, how, how often, by whom, and the reaction plan when a signal or out-of-spec result occurs; handoff to the process owner. |
| TPM | Operator-involved maintenance to prevent breakdowns; overall equipment effectiveness (OEE). |
| Visual factory | Andon lights, shadow boards, floor markings, kanban and status boards that make abnormal conditions obvious. |
Control chart selection
| Data | Chart |
|---|---|
| Continuous, individual readings (n = 1) | I-MR |
| Continuous, small subgroups (typically n = 2–9) | X̄-R |
| Continuous, larger subgroups (typically n ≥ 10) | X̄-s |
| Proportion defective (constant or varying n) | p |
| Number defective, constant n | np |
| Count of defects, constant area of opportunity | c |
| Defects per unit, varying area of opportunity | u |
Run rules
A chart can signal a special cause even when every point is inside the 3σ limits. The Western Electric rules:
- 1 point beyond 3σ
- 2 of 3 consecutive points beyond 2σ on the same side
- 4 of 5 consecutive points beyond 1σ on the same side
- 8 consecutive points on one side of the center line
The Nelson rules, used in many software packages, use 9 points on one side instead of 8 and add trends (6 points steadily rising or falling) and other patterns. Use whichever set the question names.
OEE (TPM)
OEE = Availability × Performance × Quality 0.90 × 0.95 × 0.99 = 0.846 (84.6%)
Reaction plans
Common exam traps
- Control limits ≠ specification limits. Control limits come from process data (voice of the process); spec limits from the customer. A process can be in control and still make out-of-spec product.
- I-MR for n = 1. X̄-R needs subgroups of at least 2.
- Do not adjust on common cause variation. Tampering increases variation.
- Handoff is to the process owner, with an active control plan, updated procedures and trained people; the Green Belt confirms gains hold before closing.
Learn pages for this section
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