Quality, counted and costed.
First pass yield, rework and scrap, defects per million, process capability and the cost of poor quality, from your own figures.
How it is worked outHow quality is worked out.
Counts of units and defects give the yields and the rates per million; the tolerance and the spread of the measured values give the capability; the cost model puts a price on it all.
- First pass yield, rework and scrap
Units good the first time, reworked or scrapped, each ÷ units produced.
ISO 22400-2 defines first pass yield, rework ratio and scrap ratio among its quality indicators.
- Defective parts per million
Units not good the first time ÷ units produced × 1.000.000.
- Defects per million opportunities
Defects ÷ (units × opportunities per unit) × 1.000.000.
As ISO 13053-1 defines it for Six Sigma. It counts defects, not defective units, so a unit with two defects counts twice.
- Cp and Cpk
Cp = (upper limit − lower limit) ÷ 6σ. Cpk = the distance from the mean to the nearer limit ÷ 3σ.
The capability indices of ISO 22514-4, for a stable process whose values are normally distributed. Cpk equals Cp only when the process is centred.
- Out of tolerance, expected
The share of the normal distribution beyond each limit, × 1.000.000.
- Cost of quality
Prevention + appraisal + internal failure + external failure; the two failure costs are the cost of poor quality.
The prevention, appraisal and failure model of British Standard BS 6143-2.
The figures are yours and the formulas are standard. Capability holds only for a stable process; values measured on the line show whether it is.
Sources
- ISO 22400-2:2014, Automation systems and integration: key performance indicators for manufacturing operations management, Part 2: definitions and descriptions. International Organization for Standardization. iso.org
- ISO 13053-1:2011, Quantitative methods in process improvement: Six Sigma, Part 1: DMAIC methodology. International Organization for Standardization. iso.org
- ISO 22514-4:2016, Statistical methods in process management: capability and performance, Part 4: process capability estimates and performance measures. International Organization for Standardization.
- BS 6143-2:1990, Guide to the economics of quality: prevention, appraisal and failure model. British Standards Institution. knowledge.bsigroup.com