Quality Planning, Assurance, and Control
Quality questions separate process assurance from product inspection and ask which tool answers which question. Grade and quality are different, and 'within specification' is not the same as 'in control'.
Core rules
- Quality is the degree of conformance to requirements; grade is a category for the same function. Low grade can be acceptable; low quality is always a problem.
- Cost of quality: prevention (training, process design), appraisal (testing, inspection), internal failure (scrap, rework), external failure (warranty, recalls, lost business). Prevention is cheapest; external failure is dearest.
- Quality planning outputs: quality management plan, quality metrics, quality checklists. Assurance is process-focused and produces change requests and improvements; control is product-focused and produces verified deliverables and change requests.
- Tools: cause-and-effect for root cause, Pareto for the vital few, control charts for stability over time, flowchart for process, histogram for distribution, scatter diagram for correlation, checklists for consistency.
- Control chart signals: a point outside the control limits, or seven consecutive points on one side of the mean, means the process is out of control even if every point is within specification.
Worked example
A control chart of defect rate shows every point inside the specification limits, but the last seven points sit above the process mean. The correct call is that the process is out of statistical control and needs investigation for an assignable cause, despite every unit having passed. The trap answer is 'no action needed because all points are within tolerance'. Specification limits come from the customer; control limits come from the process's own variation.
Traps
Calling points inside specification 'in control'. Using assurance for inspection. Treating grade as quality. Funding appraisal when prevention would remove the defect source. Letting the team gold-plate deliverables.