How to measure process quality? Is DPPM the correct metric?

J

juan_mexia

#1
Hello all,
I've been challenged by my boss to review if what we are doing to measure our process in correct.
Currently we are using DPPM. The problem I have with the way we do it right now is that we calculate DPPM = total number of defects / total number of produced units * 1 000 000. This way we can have more defects than produced units so the DPPM number will be higher than 1 000 000.

For example, if I want to build 10 cables with a terminal and the machine that crimps a terminal into a cable throws away 10 terminals during the process then I will have 10 defects and 10 produced units so the DPPM will be 1M.

Is this correct? It seems to me that this should be trated in a different way.

What do yo think about this? Any help will be appreciated

Thanks,
JP
 
Elsmar Forum Sponsor

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#2
Yes, if you attempt to produce 10 items, and all 10 are rejected, then yes, the failure rate is 1 million per 1 million.

However, DPPM is not a very high quality indicator of process quality. It is a go - no go piece of data. An item just shy of the specification is "good" and an item just on the other side is "bad". DPPM offers the ability to lump a bunch of data together, but I would refer proponents to the following road sign (http://en.wikipedia.org/wiki/File:New_cuyama.jpg)

Much better are control charts measuring the actual data quantities related to the specification to see how much variation there is. It is also possible to accumulate across processes by using techniques such as chi square or Hotelling's T square (http://en.wikipedia.org/wiki/Hotelling's_T-squared_distribution)
 
J

juan_mexia

#4
Thanks for the reply Steve.

So if I have 10 defects produced during the process and I at the end have 10 good units, how the DPPM should be calculated? I understand that I had 10 defects but shouldn't the denominator of the DPPM formula need to be changed?

I don't quite understand the logic of why the process can have more defects than the total of produced units.

In other hand, I do not believe we are going to change from DPPM to other quality tool to monitor our processes (at least not soon), so I am looking a more reasonable way to calculate the DPPM level, if there is.

Regards,
JP
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#5
Generally the DPPM model is used for - I attempted to make a product, and I could not send the product to the customer for a gross failure.

So, if I attempted to make 20 items, and only 10 could be shipped to the customer, this is a DPPM of 500,000 (1 of 2).

When we talk defects though, not all defects are the same. Perhaps you made an item and there was a scratch in the paint. If either you shipped it anyway figuring it would not affect the customer, or you buffed out the scratch and repainted the item, this is not a complete failure of the product.

Now, it is well worth while keeping track of minor defects, and the rework costs to fix the minor defects and ship them on to the customer.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#6
It actually sounds like the metric you are looking for is Rolled Throughput Yield (RTY). this is used to track defects internally to the manufacturing operation while DPPM is used for defects found post shipping. It is especially well suited to products that take more than a day to produce and can have multiple defects at multiple steps of the process...

RTY includes defects that are reworked and those that result in scrap. RTY will vary from 0 to 100%.

The two best articles on how to calculate this are:

Tom Pyzdek's Article "Yield the Right Way" in the March 2000 issue of Quality Digest

Spencer Grave's article "Six Sigma Rolled Throughput Yield" in the December 2001 issue of Quality Engineering. this is free for ASQ members who subscribe to QE. If you don't it's $10 - much cheaper than buying ad hoc from QE...
 
Thread starter Similar threads Forum Replies Date
J KPIs or Metrics to Measure a New Complaint Handling Process 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 5
U SI 13 - Not Every Process Requires an Efficiency Measure IATF 16949 - Automotive Quality Systems Standard 1
M How do I measure variation in my process ? Capability, Accuracy and Stability - Processes, Machines, etc. 3
P MSA requirements for Systems that Measure Process Parameters Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 3
J How to measure effectiveness of the Metrology Process General Measurement Device and Calibration Topics 5
Chennaiite How to Measure Effectiveness of the Design Validation Process? Design and Development of Products and Processes 7
D How to measure Process Capability for Homogeneous Batch Manufacturing Capability, Accuracy and Stability - Processes, Machines, etc. 4
B Process Measure for Product Development ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 17
T Process Efficiency - How to measure support processes? Clause 5.1.1 IATF 16949 - Automotive Quality Systems Standard 4
S Process Capability - TS 16949 8.2.3.1 - How to measure for various processes? Statistical Analysis Tools, Techniques and SPC 4
A How to measure Process Capability ( Cp and Cpk) Statistical Analysis Tools, Techniques and SPC 3
P EMS process Performance Measure Miscellaneous Environmental Standards and EMS Related Discussions 1
S How to measure severity if my product is designed for emergency use and failure would result in death? ISO 14971 - Medical Device Risk Management 6
A Looking for a third party to calibrate our measure equipment - South Carolina General Measurement Device and Calibration Topics 6
J Validity / outcomes measure for custom made medical device ISO 13485:2016 - Medical Device Quality Management Systems 2
D Performance specification as a Risk Control Measure, EN 14971 ISO 14971 - Medical Device Risk Management 7
D Torque fractional unit of measure; cN-m or N-cm? General Measurement Device and Calibration Topics 2
qualprod ISO 9001 5.1.1 - How to measure System effectiveness ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 4
J How to measure a Critical Characteristic that is a Basic Dimension Misc. Quality Assurance and Business Systems Related Topics 6
I Can we use pin gauges to measure an accurate Cpk ? Capability, Accuracy and Stability - Processes, Machines, etc. 9
qualprod Best criteria to measure Corrective Action effectiveness - Poor Maintenance ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 10
1 How to measure straightness on a HEX Long Bar Manufacturing and Related Processes 7
Ron Rompen Measurement Method Challenge - Measure feature #91 Inspection, Prints (Drawings), Testing, Sampling and Related Topics 9
G Measure 3 times, for calibration report? Customer Owned Reference Standard General Measurement Device and Calibration Topics 4
M Informational The ANSM decides, as a precautionary measure, to withdraw from the market macrotextured breast implants and breast implants with polyurethane-coated s Medical Device and FDA Regulations and Standards News 0
M Informational Health Canada advises Allergan of its intent to suspend its licences for Biocell breast implants as a precautionary measure Medical Device and FDA Regulations and Standards News 0
P NPR Metric - What if Customer has relaxed measure? TL 9000 Telecommunications Standard and QuEST 1
T Tape Measure, the right device for 2 or 3 decimals accuracy on prints General Measurement Device and Calibration Topics 8
M How do we document and measure a service we do not provide? IATF 16949 - Automotive Quality Systems Standard 1
M For a Type 1 Gage R&R I want to use 1 unit and measure it 30 times Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 5
M How to Measure Paper considering Variation and Humidity General Measurement Device and Calibration Topics 10
rob73 Advice/recommendations for equipment to measure a soft pvc tube - Optical Measuring? General Measurement Device and Calibration Topics 4
R Height gauge to measure short hard to access heights General Measurement Device and Calibration Topics 3
D Proper Tool to Measure Wall Thickness (ISO 3611 - Micrometers) Manufacturing and Related Processes 7
S Quality Measure for Engineering Services Misc. Quality Assurance and Business Systems Related Topics 1
A What are methods and tools to measure Workload analysis ? Human Factors and Ergonomics in Engineering 1
M Micrometer Anvil Placement to measure the diameter of Steel Wires Measurement Uncertainty (MU) 6
S Where to Get SPC Training to Measure our Processes? Training - Internal, External, Online and Distance Learning 12
M A question on Measurement System Analysis - Stability to measure got Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 2
Jim Wynne Measure Twice, Cut Once: The French Railroad Snafu World News 18
S Measure water activity for fresh raw chicken meat and giblets Food Safety - ISO 22000, HACCP (21 CFR 120) 1
O Metrology Devices to Measure the Basic Dimensions of Plastic Specimens General Measurement Device and Calibration Topics 6
L How to Measure Paint Thickness for an Enclosure Manufacturing and Related Processes 5
C Chamfer Measurements - Do you measure the height and width or the hypotenuse? Manufacturing and Related Processes 15
R Repeatability in MSA - Is repeatability a measure of Location Error Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 2
E How can I measure three true positions? GD&T basic question General Measurement Device and Calibration Topics 5
V How to use a Calibration Certificate in Uncertainty Calculation for a Test Measure Measurement Uncertainty (MU) 13
R How do companies measure QC Productivity? Quality Manager and Management Related Issues 3
V How to measure radius without radius gages APQP and PPAP 4
A Device to measure "Straightness & Deformation" of a metal that's under water Manufacturing and Related Processes 4

Similar threads

Top Bottom