Now a day’s six sigma is a very important and popular tool to optimize business or operational processes in a precise manner. Lean manufacturing and six sigma are both helpful in improvising processes but lean manufacturing is used to study the overall process and convert compact and streamlined to each process meanwhile six sigma is generally used to study the processes more precisely and minimize internal process variations.
History
- Invented by: – Bill Smith (American Engineer)
- Invented Year: – 1986
- Implemented In: – six sigma concept was first implemented in Motorola to improve manufacturing processes, secondary used in general electric, Honeywell, and caterpillar.
What is six sigma?
Six sigma is a structural technique for quality management, problem-solving, and improving business processes and their capability. Six sigma adopted a data-based approach and take decisions based on data to improve processes.
Review the attached five important things for the six sigma overview.
- Understand the problem and its basic root cause.
- Collect the factual data from processes.
- Analyze the data based on statistical quality control.
- Process implement guide on ground level.
- Process sustainability.
Six sigma mainly focuses on three categories of business processes such as quality improvement, customer satisfaction improvement, and reducing business and operational costs. In the quality improvement category, the six sigma concept defines certain criteria for defective products and also suggests measuring the same way as per listed below.
After, reviewing the attached table of process sigma level comparing with an allowable defect in DPMO (defects per million opportunities), PPM (part per million opportunities), and quality rate now time to review each parameter in detail.
DPMO
DPMO is a basic measuring rate of quality with respect to one million opportunities. (10,00,000 Nos) As per mention in the table yield is calculated based on one million opportunities. Defects mean its counts multiple entities per part. For example, each part has 6 measuring possibilities or critical to quality factors so, DPMO counts each possibility not the overall parts basis.
PPM
Part per million is also measured quality with respect to one million opportunities but the only difference is to measure quality based on the number of defective parts not including each defect per part.
After, the basic understanding of DPMO and PPM we review each with examples as per mention below.
Example
Number of parts for measurement – 100
Each part has 5 different types of defect possibility (like Defect A, B, C, D, and E) and during quality checking, check each possibility per part.
Review below attached result of the quality checker.
9 quantities have one defect per product.
4 quantities have two defects per product.
2 quantities have three defects per product.
4 quantities have four defects per product.
1 quantity having five defects per product.
No. of total defective parts = 20.
Solution: –
Part per million = (defective parts / total products produced) * 10,00,000
PPM = (20/100) * 10,00,000
PPM = 2,00,000
Shown in the attached table and review process sigma level around 2 to 3 process sigma, review the attached exact figure.
Process Sigma = 2.34
For DPMO calculation, before we proceed further we need to calculate multiple defects per product in each category.
| Defective Qty out of 100 products. | Total Defects per part. | Total defects possibility per part |
| 8 quantities have one defect per product. | 9 * 1 = 9 | 9 * 5 = 45 |
| 4 quantities have two defects per product. | 4 * 2 = 8 | 4 * 5 = 20 |
| 2 quantities have three defects per product. | 2 * 3 = 6 | 2 * 5 = 10 |
| 4 quantities have four defects per product. | 4 * 4 = 16 | 4 * 5 = 20 |
| 1 quantity having five defects per product. | 1 * 5 = 5 | 1 * 5 = 5 |
Also we check total defects possibility in good quantity = 100-20 = 80 * 5 = 400
DPMO = (Actual defects) / (Allowable defects) * 10,00,000
DPMO = (9+8+6+16+5) / (45+20+10+20+5+400) * 10,00,000
DPMO = 0.088 * 10,00,000
DPMO = 88,000
Shown in the attached table and review process sigma level around 2 to 3 process sigma, review the attached exact figure.
Process Sigma = 2.85
Now, you get a better idea about how to process sigma varies with respect to DPMO and PPM levels. Part per million measures defects based on product quantity and defects per million opportunity measures defects based on total defect possibilities per part.