Quality is a multi-faceted concept. It is not a constraint but an objective that needs to be continuously improved.
Key Quality Contributors
- W.E. Deming and Shewart (Statistical Process Control – SPC)
- J. Juran (Strategic Quality Management, Cost of Quality)
- P. Crosby (Top Management Involvement, “Quality is Free”)
- G. Taguchi (Designing Quality In)
Common and Special Causes of Variation in Quality
- Common causes are from a combination of small, purely random fluctuations. These are common to everyone in the system and are inherently a part of the process
- responsibility of management
- Special causes arise from specific circumstances
- can be responsibility of workers or management
- Use Control Charts to determine if problem is common or special
- if process measurements fall within control limits and doesn’t fall on one side of the limit too often (no “runs”), it is said to be in statistical control – only common causes afflict the process (stable)
- If a measurement falls outside the control limit, then with a high degree of probability, it’s a special cause that must be investigated.
Capability is meeting customer specifications – a large % of data points must fall within specification limits
Process capability index (Cp) is given by: Cp = USL – LSL
If Cp = 1 then you have a 3-sigma process; If Cp = 2 you have a six-sigma process (0.002 defects per million)
Total Quality Management (TQM)
- Customer satisfaction, a must for long-term success, arises from consistently meeting/exceeding customer expectations.
- Processes focus is preferred to “individual focus”
- Processes must be optimized, standardized, and documented.
- Decisions are better left to workers closer to the customer.
TQM Plan, Do, Check, Act (PDCA) Cycle
Step 1 – Analyze Situation
Step 2 – Develop Solution
Step 3 – Develop Action Plan
Step 4 – Implement & Evaluate
- Quality attitudes should not be defensive.
- Quality can be a competitive weapon.
- A capable process is one whose variability is significantly smaller than customer specified variability limits. Six-sigma programs attempt to accomplish this.
- Due to assembly stackups, quality of a composite system is significantly less than the quality of its components.