How to conduct Capability Analysis for this data?

S

sunff114

#1
The process is in statistical control, but the data is not normal. I tried all kinds of transformations, but they didn't work ( I run normality test for all transformed datasets). If you generate the dotplot or histogram of the data, you will find there are only 5 values with different frequency. The data is attached.

My questions is if it is possible for the to conduct capability analysis if the sample data is not normal.
 

Attachments

Elsmar Forum Sponsor
S

sunff114

#3
really? I used normal probability plot and normality test to check the normality. This sample is not normal. Is that what you get?
 
S

sunff114

#5
I only run the normality plots and tests for the full data. Please see the attachment in my next reply.
 
S

sunff114

#6
I have several questions about your approaches:

1. why do you run the normality test and plot for the mean of subgroups? Montgomery (1996) only mentioned to test the normality of the full data. Central limit theorem cannot apply here for capability analysis.

2. If the mean of subgroups is normal, is it safe to conduct capability analysis based on the same sample?

3. How to generate the individual points plot (the one you drawed from your attachment)? ( I am using minitab, and cannot find the method) Is there any conclusion you can draw form the individual plot?

4. I generate the normal probability plot for each subgroup, and only subgroup no. 42 and 48 didn't pass the normal test. But I am thinking the subgroup size is small (from 3 to 5), and the normality test will be more efficient for larger sample size. Please see the attachment.
 

Attachments

Statistical Steven

Statistician
Staff member
Super Moderator
#7
I have several questions about your approaches:

1. why do you run the normality test and plot for the mean of subgroups? Montgomery (1996) only mentioned to test the normality of the full data. Central limit theorem cannot apply here for capability analysis.

The estimate for the standard deviation is from the pooled within group mean. I plotted it to see if there is something different in the distribution of the means. Just exploratory.

2. If the mean of subgroups is normal, is it safe to conduct capability analysis based on the same sample?
The issue of "safe" is a relative term. A slight departure from normality of the individuals that goes away after taking the subgroup means is probably a lower risk than if the individuals and the means are not normal. Again ,slight departures in my opinion are minimal in the impact.

3. How to generate the individual points plot (the one you drawed from your attachment)? ( I am using minitab, and cannot find the method) Is there any conclusion you can draw form the individual plot?

One is from the test of normality (K-S) and the other is a graphical normality plot (Graph|probability plot)

4. I generate the normal probability plot for each subgroup, and only subgroup no. 42 and 48 didn't pass the normal test. But I am thinking the subgroup size is small (from 3 to 5), and the normality test will be more efficient for larger sample size. Please see the attachment.
BINGO! Doing a test of normality on such a small sample size of 5 is inefficient. There are some methods for normality tests when sample size is small, but they are just not reliable.
 
B

Barbara B

#8
If the (subgroup) sample size is 30 or less, the deviations from the distribution (like normal) have to be big to reject the distribution assumption. But for a capability analysis the data itself and not the data in subgroups or means of subgroups have to follow a specific distribution (most common normal distribution).

Before starting with the capability analysis itself, did you evaluate your measurement system? The resolution seems to be small as there are only five different values (2.8, 2.9, ..., 3.2). So the outcome of the process could be normally distributed but the resolution of the measurement system cut off important decimal places and therefore you cannot see a normal distribution in your data. (And maybe there isn't one at all.)

To get back to your initial question if a capability analysis could be done with this data: Technically yes, but the results won't give you reliable informations about the performance of your process. You have enough data (n=115 in 23 subgroups of size 5), but too little information within the data (probably due to a resolution problem with the measurement system).

Best regards,

Barbara
 
S

sunff114

#9
Thank you, Barbara!

I agree with you. I will still conduct capabilty analysis with some uncertainty.

Is it true that the real data is always messier/complicate than that from textbook?
 
A

Al Dyer

#10
I realize this sounds simple. Do you have a problem with scrap/customer rejections, last piece audit results???

Thanks,

Al...
 
Thread starter Similar threads Forum Replies Date
M How To Conduct Machine Capability Of An Electrical Discharge Machining (edm)? Statistical Analysis Tools, Techniques and SPC 1
M How to Conduct a Machine Capability (Cmk) Study Statistical Analysis Tools, Techniques and SPC 27
E Minumim number of parts to conduct the Process Capability Study APQP and PPAP 18
S Requirement to Conduct New Shelf-life Testing? (re-do testing for design change) EU Medical Device Regulations 3
M Informational US FDA Draft Guidance – Patient Engagement in Design and Conduct of Medical Device Clinical Investigations Medical Device and FDA Regulations and Standards News 0
B Is it necessary to conduct product audit of entire product which is manufactured in the company as per IATF IATF 16949 - Automotive Quality Systems Standard 4
C How frequent to conduct Clinical / Performance Evaluation? CE Marking (Conformité Européene) / CB Scheme 4
S Script of R software to conduct Nested GRR for Destructive Testing Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 0
J ISO 9001:2008 - Can I still conduct Internal Audits in my company? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 7
J Can I conduct Internal Audit for combined ISO 9001, ISO 13485 and ISO 14001? Internal Auditing 37
R How do I conduct my API Q1, ISO compliant internal audits? Internal Auditing 1
P Can internal ISO 17025 Auditor conduct ISO 9001 Audit ISO 17025 related Discussions 7
S CMDCAS ISO13485 - Bilateral agreement with SAI global to conduct Certification Audits ISO 13485:2016 - Medical Device Quality Management Systems 7
S Conduct Customer Satisfaction Survey for every submitted Bid? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 3
T Conduct a fractional factorial 2^13-5 experiment with Minitab Using Minitab Software 4
M Help me understand how to conduct a Chi Square Test Statistical Analysis Tools, Techniques and SPC 3
N How do I conduct Management Review Meetings? Management Review Meetings and related Processes 34
R Can our Supplier's Quality Mgr. conduct Supplier Audit on behalf of manufacturer? 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 4
F Tips, Guidelines and Checklist to conduct Audit of Supplier & Subcontractor Document Control Systems, Procedures, Forms and Templates 5
L How to conduct a Quality Audit based on ASME BPVC VIII-1 requirements Various Other Specifications, Standards, and related Requirements 4
Y FDA - Does anyone have conduct the validation on GR&R worksheet before? Qualification and Validation (including 21 CFR Part 11) 7
I Code of Ethics and Conduct wanted Document Control Systems, Procedures, Forms and Templates 3
C When is the Best Time to Conduct Internal Audits Internal Auditing 22
Q Tips on how to conduct a good Internal Audit Opening Meeting Internal Auditing 19
D How to conduct a Gage R&R on DC Tools to Torque Bolts Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 5
G How to conduct a Toy Recall (College Question) Customer and Company Specific Requirements 2
N How do I conduct Risk Management for my company? ISO 13485:2016 - Medical Device Quality Management Systems 7
K How to conduct a QCC project Misc. Quality Assurance and Business Systems Related Topics 2
N Tyvek Pouch Seal - Pressure needed to conduct Vacuum Seal Leak test? 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 2
J How to Write the Planned Interval to Conduct Internal Audit, Management Review, etc.. ISO 13485:2016 - Medical Device Quality Management Systems 3
R How to conduct the Threshold Test of Mobile Equipment specified in 3rd edition? IEC 60601 - Medical Electrical Equipment Safety Standards Series 35
K Can auditors freely conduct confidential interviews, or consent is necessary? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 22
P How to conduct ILCP (inter-laboratory comparison programme) to get ISO 17025 ISO 17025 related Discussions 3
M Do we need to conduct Vendor (Supplier) Evaluation? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 17
K When, whom, how to conduct hazard analysis of PRP?s, OPRP?s and CCP?s? Food Safety - ISO 22000, HACCP (21 CFR 120) 1
S Can a suspended CB still conduct Surveillance Audits? Registrars and Notified Bodies 4
J My mentors insist I use hotel's employee manuals and codes of conduct for my... Service Industry Specific Topics 3
somashekar Sustainability audit per EICC Supplier Code of Conduct Sustainability, Green Initiatives and Ecology 14
Marc Apple Enforces "Supplier Code of Conduct" After Child Labor Discovery After Work and Weekend Discussion Topics 13
B How to Conduct a Review of Customer Order (Contract Review)? Contract Review Process 5
V How many years of experience is needed to conduct the internal audit in my company? General Auditing Discussions 4
Sidney Vianna Can CB's (certification bodies) conduct internal and 2nd party audits? Registrars and Notified Bodies 12
G How to conduct internal audits? Internal Auditing 52
M ISO 17025 Audit check list to conduct horizontal and vertical audits ISO 17025 related Discussions 10
D How to practically-conduct, post production phase ISO 13485:2016 - Medical Device Quality Management Systems 1
S Multilevel Design in Minitab to conduct a DOE - 4 factors and each has 5 levels Design and Development of Products and Processes 4
F How to conduct CMMI appraisal Class C Software Quality Assurance 4
P How to conduct MSA for Rotation Balancing Tester Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 1
H When to conduct a MSA (Measurement Systems Analysis) study Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 5
R Hypothetical Question: How do you conduct a Stability Analysis... Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 5

Similar threads

Top Bottom