DPMO (Defects Per Million Opportunities) vs. RTY (Rolled-Throughput-Yield)

S

Sushil

#1
I need some help in understanding DPMO Vs RTY. See attached Presentation slide which I downloaded from Web. Can I relate DPMO to RTY ?. What's the rationale behind this. Is this correct ?. I did some manual calculations myself in one of the areas we are working on, this does'nt seem to be true. For my case I did the RTY calculation based on Poisson's equation & probability of zero defects as e ^(-DPU). Can someone guide me on this and how can I explain relation between RTY and DPMO..? How can I calculate RTY based on DPMO data ?.
 

Attachments

Elsmar Forum Sponsor
W

wmarhel

#2
Why would you want to?

Sushil said:
Can I relate DPMO to RTY ?. What's the rationale behind this. Is this correct ?
For RTY (Rolled-Throughput-Yield) it doesn't matter whether there are 10 defects or just a single defect. It failed to meet to the necessary quality requirements to move onto the next operation/work center/etc.

I'm not a huge fan of DPMO as it relates to the manufacturing process, it's more of a tool to used during the design and re-design of new products/processes. A defective product is a defective product, irregardless of whether three things don't work on it, or just one. Sometimes it feels that there are engineers out there who believe that if it isn't broken.....there just aren't enough features on it.

Here's a thread on DPMO which may or not help: DPMO calculation for Testing stations- ideal way? High volume PCB manufacture

Wayne
 
J

James Oldham

#3
Sushil,

You can convert RTY into DPMO and back again - they are just different ways of showing the same number. RTY as a % is # good/total, therefore 1-RTY is the % defective. Since % defective is technically "defects per hundred", just multiply this number times 1,000,000 to get DPMO.

Is the glass half full or half empty? This is another one of those cases why lots in the forum shun Six Sigma. Just focus on the # of defects and fix the problems - the form in which you report the number doesn't matter.
 
S

Sushil

#4
James Oldham said:
Sushil,

You can convert RTY into DPMO and back again - they are just different ways of showing the same number.
James.. pardon me

I am still confused. I do not understand why DPMO % is being related and termed as RTY. I did some study from a station in floor and following were my findings, which confuses me more..
total Defects =500
Total Opps =7787260
DPU = 0.41
RTY =e^-DPU=66.4%
DPMO = 64.21

as per table attached in my first post
DPMO %(RTY) for 64.21DPMO will be 99.99353%

why is this difference..? and is there any issue terming DPMO% as RTY ? can it be done..

:confused:
 
G

gvalenti9

#5
DPMO and RTY

Hopefully I can explain this clearly.

total Defects =500
Total Opps =7787260
DPU = 0.41

m = number of opportunities/unit = 6383. I calculated this using the total # of defects (500) divided by the DPU to derive that the data was based on 1220 units then divided the total opps by the # of units.

RTY =e^-DPU=66.4%
DPMO = 64.21

Yna = Normalized yield = RTY ^ (1/m) = 0.664^(1/6383) =.99993585 which matches up to 99.99353% that you calculated

DPMO can be calculated from the Yna or from actual yield data
DPMO = (1-Yna)*10^6 = (1-0.9999353)*10^6 = 64.15
DPMO = # defects/# opportunities *10^6 = 500/7787260*10^6 =64.21

So then, why does RTY not equal DPMO and DPMO not equal DPMO?

DPMO = # defects/# opportunities *10^6 is an actual calculation based are units processed, number of defects found, and number of real opportunities per unit.

DPMO = (1-Yna)*10^6 is based on the Poisson distribution used to calculate RTY. RTY = e^-DPU assumes defects are distributed randomly and are assumed to follow a Poisson distrubtion. Yna, therefore, reflects the assumed Poisson distribution used to calculate RTY . As the defect rate falls below 10%, Yna and DPMO converge. The data that you have shows them converging quite nicely. It appears that you were only missing the one step DPMO = (1-Yna)*10^6

The benefit of the DPMO is that it normalizes the data so that assemblies of different complexities/defect opportunities can be compared objectively. The benefit of RTY is that if there is an issue with yields, it becomes immediately obvious.
 
Last edited by a moderator:
Q

QualityKangaroo

#6
Sushil or anyone else out there,

I am responding to why DPMO can/can not be related to RTY.

I have found that one major difficult (as pointed out above) is the normal distribution assumption... as you may know, even if an overall process exhibits normality certain process steps do not. It is actually helpful to compare CpK (capability) to FTY (in RTY sense) to check this assumption... though not foolproof it may show you some interesting things happening in the process. However, the main reason I have added to this thread: No one seems to have brought up the point that DPMO looks at defects which, let us assume, are properly recorded and assigned a cause and process step. We can usually pull this data with ease. However RTY also pulls in "rework". It is sometimes dangerous to assume that all "out-of-spec"/"out-of tolerance" are either recorded as waste or reworked. Sometimes in-spec product is re-worked for an number of reasons (good or bad) and is therefore not captured in the DPMO umbrella. This is actually why I prefer RTY when looking at a process - it can direct you to little hidden rework zones that DPMO often misses. So, in conclusion DPMO misses out on some of the rework and therefore gives an incomplete picture of the reality on the shop floor.

Hope this helps.

Q. Kangaroo
 

Jim Wynne

Leader
Admin
#7
Sushil or anyone else out there,

I am responding to why DPMO can/can not be related to RTY.

I have found that one major difficult (as pointed out above) is the normal distribution assumption... as you may know, even if an overall process exhibits normality certain process steps do not. It is actually helpful to compare CpK (capability) to FTY (in RTY sense) to check this assumption... though not foolproof it may show you some interesting things happening in the process. However, the main reason I have added to this thread: No one seems to have brought up the point that DPMO looks at defects which, let us assume, are properly recorded and assigned a cause and process step. We can usually pull this data with ease. However RTY also pulls in "rework". It is sometimes dangerous to assume that all "out-of-spec"/"out-of tolerance" are either recorded as waste or reworked. Sometimes in-spec product is re-worked for an number of reasons (good or bad) and is therefore not captured in the DPMO umbrella. This is actually why I prefer RTY when looking at a process - it can direct you to little hidden rework zones that DPMO often misses. So, in conclusion DPMO misses out on some of the rework and therefore gives an incomplete picture of the reality on the shop floor.

Hope this helps.

Q. Kangaroo
Why would conforming product need to be reworked?
 
Q

QualityKangaroo

#8
One reason (though not a good one) that I often run into is that an operator attempts to react to an existing condition communicated by another team member (downstream)... it may not even be something we currently measure but which makes the job easier down the process. Secondly, and perhaps more troubling, is that an operator establishes their own reaction limits. Most of the time this can be addressed by proper coaching and follow-up training, but I have visited many plants with the operator is prideful to a detriment - they chase a target and cause greater variation AND often rework acceptable product. One simple example: I'm making cookies and I don't like diameter (even though its in spec)... I take the dough and replace it in to a mixer. The mentality expressed is often, "I want to better, what's wrong with that?"... suddenly you have a less efficient process (and likely one with more variation considering the operator attempt to make the diameter "better"). I hope this clarifies:thanx:
 
Thread starter Similar threads Forum Replies Date
optomist1 DPM (Defects per Million) vs. DPMO (Defects per Million Opportunities) Capability, Accuracy and Stability - Processes, Machines, etc. 4
T DPMO (defects per million opportunities) - Key word being OPPORTUNITIES Six Sigma 9
Marc Definition DPMO - Defects Per Million Opportunities Definitions, Acronyms, Abbreviations and Interpretations Listed Alphabetically 1
J Definition DPMO and PPM - Differences - Defects Per Million Opportunities and Parts Per Million? Definitions, Acronyms, Abbreviations and Interpretations Listed Alphabetically 7
F DPMO (Defects Per Million Opportunities) Calculation Statistical Analysis Tools, Techniques and SPC 5
S Measurment of DPU/DPMO in Assembly process Capability, Accuracy and Stability - Processes, Machines, etc. 1
Z DPMO? Metrics for Engineering Drawings Six Sigma 1
D Siemens MES software DPMO Calculation Six Sigma 4
B Calculating Combined DPMO and Sigma Level for Two or More Different Work Areas Six Sigma 3
R Six Sigma / DPMO Related Inquiry - A 6 Sigma related Statistical Sampling question Six Sigma 2
1 What are TOP, DPU, DPO, DPMO? Mult-CTQ (discrete) ZST - Is this a probability? Six Sigma 1
S Confusion DPU, PPM & DPMO vs. Sigma Level - ASQ Six Sigma Black Belt hand book Six Sigma 14
ScottK Realistically - What kind of DPMO and sigma for a $5.00 part? Six Sigma 30
Y Definition DPPM - Quality Indicator? Using DPPM instead of DPMO as our OQA quality indicator Definitions, Acronyms, Abbreviations and Interpretations Listed Alphabetically 4
S PPM vs. DPMO - Confused on how to use Six Sigma 2
B How to decide the ?U? to calcuate DPU and "opportunity" of DPMO Six Sigma 8
B How to define and Calculate the DPU, DPMU, DPMO, Yield, Rolled Throughput Yield (RTY) Manufacturing and Related Processes 3
J How to calculate the sigma level of a process and Relationship of DPMO Six Sigma 7
S DPMO calculation for Testing stations- ideal way? High volume PCB manufacture Six Sigma 8
M Industry DPMO Standards - Contract Electronics Manufacturing processes Statistical Analysis Tools, Techniques and SPC 2
T Calculating DPMO for Exponential Distributions Statistical Analysis Tools, Techniques and SPC 4
J Converting DPMO to SIGMA - Seeking Forumula Statistical Analysis Tools, Techniques and SPC 16
L Corrective and Preventive Actions aligned to design related defects. After Work and Weekend Discussion Topics 5
Q Product Quality vs Product Defects Manufacturing and Related Processes 4
A % of defects on the whole batch based on result from inspection under AQL Level II Inspection, Prints (Drawings), Testing, Sampling and Related Topics 6
C Random defects vs systematic (designed in) errors or faults. Design and Development of Products and Processes 1
A AQL - How to count samples with defects for each defect class AQL - Acceptable Quality Level 17
B IATF 16949 Cl. 8.6.6 - Acceptance Criteria - Zero Defects Attribute Data Sampling IATF 16949 - Automotive Quality Systems Standard 2
J Validating a stopped test due to never finding any defects Inspection, Prints (Drawings), Testing, Sampling and Related Topics 2
Q PPM - Filtering Defects out of calculation Quality Tools, Improvement and Analysis 5
S Determining sample size for inspection to achieve x% confidence re defects Misc. Quality Assurance and Business Systems Related Topics 10
R Classification of Major & Minor Defects AQL - Acceptable Quality Level 10
S Understanding, Analysis and Monitoring Quality Defects on Composite Components Statistical Analysis Tools, Techniques and SPC 3
K Is there any specific Standard on visual defects for extrusion and machined parts ? Quality Tools, Improvement and Analysis 6
P What are the causes of painting defects? Manufacturing and Related Processes 2
T Repeated Inspection - Probability of Visual Inspection catching Defects Inspection, Prints (Drawings), Testing, Sampling and Related Topics 2
T How many complaints, defects and non-conformances are typical? Customer Complaints 5
P Monitoring Production Defects - Three simple questions Six Sigma 3
S Are P-charts for a Particular Defect or can Different Defects be counted in one? Statistical Analysis Tools, Techniques and SPC 2
E Post Machining Casting Defects with MIL-STD-2175 Requirements - Responsibility Manufacturing and Related Processes 9
L What is the Cpk for Zero Defects Six Sigma 4
S Allowed Faults (Defects) on Plastic Parts Inspection, Prints (Drawings), Testing, Sampling and Related Topics 6
G Sample Size for 2900 ppm Visual Defects Inspection Requirement Inspection, Prints (Drawings), Testing, Sampling and Related Topics 4
T 10 CFR Part 21 - Reporting of Defects and Noncompliance Various Other Specifications, Standards, and related Requirements 6
Crusader "Defects" Visual Aid: To Control or To Not Control? Document Control Systems, Procedures, Forms and Templates 8
I Integrated Management System - Nonconforming Service/Product Defects ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 4
T Supplier Spray Process Audit - Fish Eye, Spray Dust and Fiber Dust Defects Manufacturing and Related Processes 8
I Cpk < 1.33 - How do I estimate actual defects? Statistical Analysis Tools, Techniques and SPC 7
R Suppliers Defects found during Production Process - Supplier Scorecards Quality Tools, Improvement and Analysis 6
N Causes of Disbond Defects in Composites Manufacturing Process Manufacturing and Related Processes 7

Similar threads

Top Bottom