Unit 3: Acceptance sampling and quality assurance
Quality Toolkit for Managers notes · PTU syllabus (MBA 954-18)
On this page
Unit summary
Sampling and assurance systems keep quality high without inspecting everything. This unit covers methods of taking samples, the operating characteristic curve, single, double, multiple and sequential sampling plans, principles of quality assurance, the quality manual, field complaints, types of quality audit, and QC vs QA.
After this unit you can
- Explain sampling methods and the OC curve
- Compare single, double, multiple and sequential sampling plans
- Explain quality assurance, the quality manual and field complaints
- Explain quality audits and distinguish QC from QA
PTU syllabus topics
- Methods of taking samples
- operating characteristics curve
- single/double/multiple/sequential sampling plans
- quality assurance principles
- quality manual
- field complaints
- quality audit types
- QC vs QA
Focus
Finding defects in products
Preventing defects in processes
Approach
Reactive: inspection, testing
Proactive: systems, audits
Responsibility
QC team
Everyone
Topic 1
Methods of taking samples
- Random sampling: every item equally likely to be chosen (random numbers).
- Stratified sampling: samples from each stratum (shift, machine, supplier lot).
- Systematic sampling: every k-th item.
- Representative sampling avoids bias from picking only the top layer of a lot.
Topic 2
Acceptance sampling and the OC curve
Acceptance sampling inspects a sample from a lot to accept or reject the whole lot.
Single sampling
One sample of n; accept if defectives ≤ c
Simple; larger average sample
Double sampling
Second sample if the first is inconclusive
Smaller average sample
Multiple sampling
Several small samples in sequence
Smallest average sample, complex
- OC curve: probability of accepting lots of different quality. AQL (acceptable quality level) and LTPD (lot tolerance percent defective) define producer's risk (α) and consumer's risk (β).
- Sequential sampling: items inspected one at a time; after each, decide to accept, reject or continue — smallest average sample.
- OC curve: an ideal plan would accept all lots better than AQL and reject all worse; real OC curves slope, with producer's risk (α, usually 5%) at AQL and consumer's risk (β, usually 10%) at LTPD.
- Standards: IS 2500, ISO 2859 (attributes), MIL-STD-105E heritage.
Topic 3
Principles of quality assurance
- Quality assurance: planned and systematic activities to provide confidence that quality requirements will be fulfilled.
- Principles: prevention over detection, process approach, documented procedures, customer focus, management responsibility, continual improvement, supplier quality, training, evidence-based decisions.
Topic 4
The quality manual
- Quality manual: top-level document describing the organisation's quality management system.
- Contents: scope, quality policy and objectives, organisation and responsibilities, processes and their interactions, references to procedures and work instructions, document control. (ISO 9001:2015 no longer mandates a manual but most firms keep one.)
Topic 5
Field complaints
- Field complaints: customer complaints about products in use — valuable data on real performance.
- 1
Receive and register
- 2
Acknowledge the customer
- 3
Investigate and find root cause
- 4
Corrective and preventive action (CAPA)
- 5
Resolve and respond
- 6
Analyse trends and feed back to design and production
Topic 6
Quality audits
By party
First-party (internal), second-party (customer audits supplier), third-party (certification body)
By scope
System audit, process audit, product audit
Others
Compliance audit, follow-up audit, supplier audit
- Audit process: plan, prepare checklists, opening meeting, collect evidence, report findings (non-conformities), corrective action, follow-up.
Topic 7
QC vs QA
Focus
Product — detecting defects
Process — preventing defects
Approach
Reactive, inspection and testing
Proactive, systems and procedures
Responsibility
Inspection or QC staff
Everyone, led by management
Tools
Control charts, inspection, testing
Audits, standards, documentation, training
Key terms
- Sequential sampling
- Item-by-item decision to accept, reject or continue
- OC curve
- Probability of accepting lots of varying quality
- Quality manual
- Document describing the quality management system
- CAPA
- Corrective and preventive action
- Third-party audit
- Audit by an independent certification body
Quick revision
- Random, stratified, systematic sampling.
- Single, double, multiple, sequential plans; OC curve; AQL, LTPD, α, β.
- QA principles; quality manual contents.
- Field complaint handling; CAPA.
- First-, second-, third-party audits; QC vs QA.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.What is stratified sampling?
- Q2.What is an OC curve?
- Q3.Distinguish double and sequential sampling.
- Q4.What does a quality manual contain?
- Q5.Name three types of quality audit.
- Q6.Distinguish QC and QA.
Long-answer questions
- Q1.Explain acceptance sampling plans and the OC curve.
- Q2.Explain the principles of quality assurance and the quality manual.
- Q3.Explain the handling of field complaints and types of quality audit.
- Q4.Compare quality control and quality assurance.
Stuck on this unit?
Message SBS on WhatsApp for help with Quality Toolkit for Managers, or to ask about studying MBA at Synetic.