Quality management: SQC, TQM, Six Sigma and ISO 9000
Quality is built in, not inspected in. SQC uses control charts and acceptance sampling; TQM, kaizen, benchmarking, Six Sigma and ISO 9000 make quality everyone's continuous job.
Quality gurus and their ideas
| Guru | Key contribution |
|---|---|
| W. Edwards Deming | 14 points for management; PDCA (Deming or Shewhart) cycle; reducing variation; seven deadly diseases |
| Joseph Juran | Quality trilogy (planning, control, improvement); 'fitness for use'; Pareto's vital few and trivial many |
| Philip Crosby | Zero defects; 'Quality is Free' (1979); quality as conformance to requirements; cost of quality |
| Kaoru Ishikawa | Cause-and-effect (fishbone) diagram; quality circles; seven basic QC tools |
| Armand Feigenbaum | Total quality control; the 'hidden plant' lost to rework |
| Genichi Taguchi | Quality loss function; robust design |
| Shigeo Shingo | Poka-yoke (mistake-proofing); SMED (quick changeover) |
| Walter Shewhart | Control charts; statistical process control |
Control charts
| Chart | Data type | Tracks | Based on |
|---|---|---|---|
| x̄ chart | Variables (measured) | Process average | Normal; limits x̿ ± A₂R̄ |
| R chart | Variables | Process spread (range) | Limits D₃R̄ and D₄R̄ |
| p chart | Attributes (good or bad) | Fraction defective | Binomial; p̄ ± 3√[p̄(1 − p̄) ÷ n] |
| np chart | Attributes | Number defective | Binomial |
| c chart | Attributes (count of defects) | Defects per unit | Poisson; c̄ ± 3√c̄ |
| u chart | Attributes | Defects per unit when the unit size varies | Poisson |
Chance vs assignable causes
Worked example 1: x̄ and R chart limits
Samples of 5 bearings give a grand mean x̿ = 50 mm and an average range R̄ = 4 mm. For n = 5, A₂ = 0.577, D₃ = 0 and D₄ = 2.114. Find the control limits.
- x̄ chart: UCL = 50 + 0.577 × 4 = 50 + 2.308 = 52.308 mm; LCL = 50 − 2.308 = 47.692 mm.
- R chart: UCL = D₄R̄ = 2.114 × 4 = 8.456 mm; LCL = D₃R̄ = 0 × 4 = 0.
x̄ chart: 47.692 to 52.308 mm (centre 50). R chart: 0 to 8.456 mm (centre 4).
Worked example 2: p chart limits
20 samples of 100 units each contain 80 defectives in all. Find the limits of the p chart.
- p̄ = 80 ÷ (20 × 100) = 0.04.
- σp = √[0.04 × 0.96 ÷ 100] = √0.000384 = 0.0196 (4 decimal places).
- UCL = 0.04 + 3 × 0.0196 = 0.0988. LCL = 0.04 − 0.0588 = −0.0188, which is set to 0.
Centre 0.04, UCL ≈ 0.0988, LCL = 0 (a negative limit is taken as zero).
Acceptance sampling
- Inspect a random sample from a lot. Accept the lot if the defectives are at most the acceptance number c; otherwise reject it. Used when 100% inspection is costly or destructive.
- Plans: single, double, multiple and sequential sampling.
- The operating characteristic (OC) curve shows the probability of accepting lots of each quality level.
- Producer's risk (α): a good lot, at the AQL, is rejected. Consumer's risk (β): a bad lot, at the LTPD, is accepted.
- AOQL is the worst average outgoing quality after rejected lots are screened. Dodge and Romig published standard sampling tables.
TQM and continuous improvement tools
| Concept | Core idea | Linked with |
|---|---|---|
| TQM | Customer focus, involvement of every employee, a process approach and continuous improvement across the whole organisation | Deming, Juran, Feigenbaum |
| Kaizen | Small, continuous improvements by everyone, every day, at the workplace (gemba) | Masaaki Imai (1986); Toyota |
| Quality circle | A small group of workers from the same area meets voluntarily and regularly to solve work problems | Ishikawa; Japan, 1962 |
| Benchmarking | Compare processes with the best in class and adopt better practices; internal, competitive, functional or generic | Xerox (1979); Robert Camp |
| 5S | Sort, set in order, shine, standardise, sustain (seiri, seiton, seiso, seiketsu, shitsuke) | Japanese workplace practice |
| Poka-yoke | Mistake-proofing, so an error cannot happen or is caught at once | Shigeo Shingo |
| Cost of quality | Prevention and appraisal costs (good quality) vs internal and external failure costs (poor quality) | Juran, Crosby |
Six Sigma DMAIC
DMAIC improves an existing process; DMADV (define, measure, analyse, design, verify) designs a new one. Roles include Champion, Master Black Belt, Black Belt and Green Belt.
Worked example 3: DPMO
A bank processes 1,000 loan files, each with 5 chances of error (opportunities). Auditors find 50 errors. Find the defects per million opportunities.
- Total opportunities = 1,000 × 5 = 5,000.
- Defects per opportunity = 50 ÷ 5,000 = 0.01, a yield of 99%.
- DPMO = 0.01 × 10,00,000 (one million) = 10,000.
10,000 DPMO, far from the Six Sigma target of 3.4.
The ISO 9000 family
| Standard | What it is |
|---|---|
| ISO 9000 | Fundamentals and vocabulary of quality management systems |
| ISO 9001 | Requirements for a QMS; the only one used for certification |
| ISO 9004 | Guidance for sustained success beyond the basic requirements |
| ISO 19011 | Guidelines for auditing management systems |
ISO points to remember
- ISO was founded in 1947 and is based in Geneva. It first issued the ISO 9000 series in 1987. In India, BIS publishes it as IS/ISO 9000.
- Seven quality management principles: customer focus, leadership, engagement of people, process approach, improvement, evidence-based decision making and relationship management.
- Accredited third-party bodies certify firms; ISO itself does not certify.
- ISO 14001 (environment) and ISO 45001 (occupational health and safety) are separate standards, not part of the ISO 9000 family.
NET asks you to match gurus with ideas, choose the right control chart (x̄, R, p, np, c), sequence DMAIC, separate producer's and consumer's risk, and know which ISO 9000 standard is certifiable. Short p-chart or DPMO sums also appear.
Typical MCQ: “What is the producer's risk?” The probability of rejecting a good lot (at the AQL); it is α. The consumer's risk β is accepting a bad lot (at the LTPD). Trap: swapping the two.
Typical MCQ: “Which chart tracks the number of defects on each car door panel?” The c chart (Poisson). Trap: the p chart is for the fraction of defective units, not counts of defects.
Typical MCQ: “Which ISO standard are firms certified to?” ISO 9001. Trap: ISO 9000 covers only fundamentals and vocabulary.
DMAIC = 'Do Many Analysts Improve Continuously': Define, Measure, Analyse, Improve, Control. c for counts, p for proportions.