Cost of quality (CoQ)
| Category | Examples |
|---|---|
| Cost of conformance (spend to prevent defects) | |
| Prevention | Training, documenting processes, proper equipment. |
| Appraisal | Testing, inspection, audits. |
| Cost of non-conformance (spend because of defects) | |
| Internal failure | Rework, scrap — found before delivery. |
| External failure | Warranty claims, recalls, lost business — found by the customer. |
Prevention is cheaper than failure — spending on conformance reduces the larger cost of non-conformance.
Quality assurance vs quality control
| Quality Assurance (QA) | Quality Control (QC) |
|---|---|
| Process-focused | Product-focused |
| Prevent defects (proactive) | Find defects (reactive) |
| Audits, process improvement | Inspection, measurements, testing |
The seven basic quality tools
- Cause-and-effect (Ishikawa / fishbone): trace a problem to root causes.
- Flowchart: map the steps of a process.
- Check sheet: tally defects/events as they occur.
- Pareto chart: rank causes — the 80/20 rule.
- Histogram: show the distribution of data.
- Control chart: track a process against control limits over time.
- Scatter diagram: show correlation between two variables.
Control chart: rule of seven
Seven consecutive points on one side of the mean signals a non-random trend — investigate even if within control limits.
Mistakes examiners test
- Swapping QA and QC. QA = process/prevent; QC = product/inspect.
- Pareto vs histogram. Pareto is an ordered histogram ranking causes by frequency.
- Ignoring the rule of seven. A run of seven means out-of-control even within limits.
Test yourself
4 quick questions — tap an answer to check it instantly. Nothing is sent anywhere.
1. Inspecting finished products is an example of…
Answer: B. Inspection and testing of the product is Quality Control (QC).
2. Which is a cost of non-conformance?
Answer: C. Internal failure (rework, scrap) and external failure are non-conformance costs.
3. A Pareto chart helps you…
Answer: B. A Pareto chart is an ordered histogram ranking causes — the 80/20 rule.
4. Seven consecutive points on one side of the mean on a control chart means…
Answer: B. The 'rule of seven' flags a non-random trend even within control limits.
Sources
- Project Management Institute — pmi.org.
- PMI, A Guide to the Project Management Body of Knowledge (PMBOK® Guide).
Study summary only. "PMP" and "PMBOK" are marks of the Project Management Institute, which does not endorse this material.