SBS - The best value in QMS software

Analyzing Data with a Non-Normal distribution

C

Cognizant

#1
Hi folks,

Recently one of our customers were questioning about our data was not normally distributed. we were concerned about cpks. but his concern was u cant analyse the data if it is not normally distributed...

my concern how cud u get the data normally distrribution.
 
Elsmar Forum Sponsor
C

Craig H.

#2
One way to help aleviate this would be to use an X Bar and R chart instead of an XMR chart because a distribution of averages will be more normal, especially as n increases.
 

Statistical Steven

Statistician
Staff member
Super Moderator
#3
Cognizant said:
Hi folks,

Recently one of our customers were questioning about our data was not normally distributed. we were concerned about cpks. but his concern was u cant analyse the data if it is not normally distributed...

my concern how cud u get the data normally distrribution.
Did the customer use any analytical tool to prove your data is not normally distributed? It might be true that your Cpk values are biased by the nonnormality, but first you need to have a statistical test that does not accept normality. Try the Wilks-Shapiro method, Kolmogorov-Smirnov method or just a normal probability plot.
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#4
Interesting how much concern there is over normality of late.

SPC does not depend upon normality. Shewhart did simulations with several skewed distributions. Wheeler has also done work with various non-normal distributions and found SPC works even if the data are not normal.

Myself, I am not a believer in using CpK. Much better to judge from the control chart itself how the system is performing and if its results are predictable or not.

If normality absolutely is an issue, then yes you can study the data to see if it normal or not, and if really really necessary one could develop a mathematical transform in order to shift the data to normality.

Another option is to shift to non-parametric analyses.
 
Last edited:

Statistical Steven

Statistician
Staff member
Super Moderator
#5
Steve Prevette said:
Wheeler has also done work with various non-normal distributions and found SPC works even if the data are normal.
I think you meant to type if the data are NOT normal.

I do not like Cpk either, but with the 6S craze, people want a single metric that describes their process. Given that, you can report a Cpk, but I do not hold to the 1.33 or any other rule as a good process.
 
C

Cognizant

#7
NOt normal distribution

Our data failed the normal distribution on many key charactericstics...for example one part key characteric thickness .013+/-.001(inches) thickness Rockwell 50 ..our boss always orders the material at .0135 RC50 because he can't find material at .013 and RC 50...and all the data points are around .0135...this ultimately lead to a non normal distribution with 6 pack Minitab R14... and we said this is what it is nothing can be done to make the data Normal......

This is just one example ...there are many other key characteristics which are not normally distributed...we always go with Cpks and Ppks but our customers says u cannot go for Cpks and Ppks without the normality test ....then we asked are there any tools to use non normal data..he said he was not sure ...

I am sure Minitab provides solutions both for normal as well as not normal distributions....but my concern is which one to use...Our data sometimes lead to normal distributuion and sometimes to not normal distribution on some characteristics.....

Thats our major customers concern ..Y the data s not normal??????

Thanks for answering folks..I'd really appreciate it...
 

Jim Wynne

Staff member
Admin
#8
Cognizant said:
Our data failed the normal distribution on many key charactericstics...for example one part key characteric thickness .013+/-.001(inches) thickness Rockwell 50 ..our boss always orders the material at .0135 RC50 because he can't find material at .013 and RC 50...and all the data points are around .0135...this ultimately lead to a non normal distribution with 6 pack Minitab R14... and we said this is what it is nothing can be done to make the data Normal......

This is just one example ...there are many other key characteristics which are not normally distributed...we always go with Cpks and Ppks but our customers says u cannot go for Cpks and Ppks without the normality test ....then we asked are there any tools to use non normal data..he said he was not sure ...

I am sure Minitab provides solutions both for normal as well as not normal distributions....but my concern is which one to use...Our data sometimes lead to normal distributuion and sometimes to not normal distribution on some characteristics.....

Thats our major customers concern ..Y the data s not normal??????

Thanks for answering folks..I'd really appreciate it...
The fact that the data is centered around .0135 and not .0130 doesn't mean the distribution isn't normal. In fact, because you're referring to some sort of material thickness, I'd bet that the distribution is normal. So what do you mean by "not normal"?
 

Statistical Steven

Statistician
Staff member
Super Moderator
#9
Jim Wynne said:
The fact that the data is centered around .0135 and not .0130 doesn't mean the distribution isn't normal. In fact, because you're referring to some sort of material thickness, I'd bet that the distribution is normal. So what do you mean by "not normal"?
Might also fail because of a lack of gage discrimination. But, I agree noncentered in the specification does not imply nonnormality.
 
C

Cognizant

#10
Msa

I mean .0135 is not normal in the sense ...example ..I have the data of 50 pcs

20 pcs were .0133
25 pieces were .0134
5 pieces were .0135

this is not normal data at all...it fails the normality test....

I am guessing its probably the measurement system ..
It needs to be more accurate reaching upto millions 5-7 decimal places...
But I can't get that accurate data with ball mics.....I have to use some kind of optical measuring system for which my boss never agrees to buy......
 
Thread starter Similar threads Forum Replies Date
T Analyzing internal data for corrective action Internal Auditing 4
G Characterization Studies: Analyzing Data to Establish Conclusions Statistical Analysis Tools, Techniques and SPC 1
N Different Distribution Parameters when analyzing the same data in different ways Using Minitab Software 8
C Analyzing Attribute Data - Mechanical Pull Testing of Joint Statistical Analysis Tools, Techniques and SPC 15
M Transforming your Data - Analyzing a DOE Six Sigma 5
N ANOVA Help Analyzing Extrusion Data Statistical Analysis Tools, Techniques and SPC 7
MarilynJ6354 Satmetrix Net Promoter - Has anyone used this for analyzing customer survey data? Service Industry Specific Topics 4
qualprod Complying with ISO 9001:2015 - 9.1.1 b - Monitoring, measuring, analyzing and evaluation ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 2
Q Analyzing Process Bottlenecks Tools ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 16
S ANOVA/Regression - Analyzing a Dataset which contains over 4500 Points Using Minitab Software 3
W Best quadratic subset for analyzing a Response Surface Design with Minitab 15 Using Minitab Software 6
C Analyzing 3 Factors 3 Levels using Minitab Using Minitab Software 9
P Help Setting Up and Analyzing 3 Factor 2 Level Full Factorial Design for DOE Using Minitab Software 3
M Type 1 Gage R&R - Analyzing Run Chart and I-mR Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 12
M I came across this site with several analyzing tools (Excel) Statistical Analysis Tools, Techniques and SPC 4
R Analyzing Customer Satisfaction Indexes Quality Manager and Management Related Issues 1
S Chi-Square Conundrum - Analyzing tree death across 3 areas Inspection, Prints (Drawings), Testing, Sampling and Related Topics 13
C Analyzing Full Factorial Design in Minitab Using Minitab Software 5
M FMEA Interface Matrix - Analyzing risks of electronic health records system FMEA and Control Plans 4
A Analyzing Corrective/Preventive Actions and How to Track Due Dates Preventive Action and Continuous Improvement 16
D Analyzing fishbone diagram Quality Tools, Improvement and Analysis 4
A Net Promoter Score (NPS) for Analyzing the Client Satisfaction Misc. Quality Assurance and Business Systems Related Topics 3
K Process monitoring, measuring and analyzing, does this means all company processes? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 3
D Tools for analyzing 3 factors at the same time Quality Tools, Improvement and Analysis 12
Q AMS 2750 E or F Continuous Furnace TUS Data Collection AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 5
M Reduce occurrence rating based on the PMS data and customer complaint data ISO 14971 - Medical Device Risk Management 2
J Customer Complaint & SCAR, false data Nonconformance and Corrective Action 14
Brizilla Employee Data Privacy Policy - ISO 9001:2015 requirement(s)? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 6
D Safety data sheets software REACH and RoHS Conversations 2
M Data Protection and Privacy Policy - looking for a template/example EU Medical Device Regulations 1
S Non parametric test for semi-quantitative data. Statistical Analysis Tools, Techniques and SPC 5
M Disabling measurement data during fault conditions IEC 60601 - Medical Electrical Equipment Safety Standards Series 5
C EU MDR - Annex II 6.1 Pre-clinical and clinical data EU Medical Device Regulations 4
P Ppk results shown as asterisk after the transformation of Non-normal data Using Minitab Software 4
lanley liao How to correctly understand the bullet list d) of 6.3 Analysis of Data for API Spec Q1 Oil and Gas Industry Standards and Regulations 7
Steve Prevette Informational I am presenting a webinar Thursday - "Data Driven Decision Making" - 19 November 2020 Statistical Analysis Tools, Techniques and SPC 5
qualprod Best practice to ensure inputting of data in production Lean in Manufacturing and Service Industries 19
D Preservation of Electronic Data / Information Technology ISO 13485:2016 - Medical Device Quality Management Systems 5
M Comparing data from destructive testing Inspection, Prints (Drawings), Testing, Sampling and Related Topics 7
DuncanGibbons Technical Data Package vs Digital Product Definition APQP and PPAP 0
Z Putting back excluded rows/data points in a control chart Using Minitab Software 0
F General Data Protection Regulation (GDRP) CE Marking (Conformité Européene) / CB Scheme 6
Z Minitab - Updating Graph with specific data points Using Minitab Software 2
E PEMS Hazards - IEC 60601 Clause 14.6 - Internal data use - Pressure sensor IEC 60601 - Medical Electrical Equipment Safety Standards Series 3
K Transform variable data into attribute data Reliability Analysis - Predictions, Testing and Standards 24
R Clinical evaluation without clinical data - MDR Article 61(10) EU Medical Device Regulations 8
H Capability Data for Paint Thickness on Painted Parts Statistical Analysis Tools, Techniques and SPC 10
D BS EN 62304 - Medical-Relevant Data C.5 - Definition of IEC 62304 - Medical Device Software Life Cycle Processes 5
T Submitting MR Compatibility Data for 510(k) Cleared Device Other Medical Device and Orthopedic Related Topics 2
S Quality manager considering data science Quality Manager and Management Related Issues 19

Similar threads

Top Bottom