Software Calculation Errors for Cp and Cpk With Unilateral Tolerances

Caster

An Early Cover
Trusted Information Resource
#1
[font=Times New (W1)]I have looked back through the archives for Cp and Cpk on unilateral tolerances, and I still need some help.[/font]

[font=Times New (W1)]We now have a situation with our SPC software when it determines capability for unilateral tolerances. The software supplier has offered to fix this if they can get a definitive ruling on what is correct. So here’s your chance.[/font]

[font=Times New (W1)]Here is an example.[/font]

[font=Times New (W1)]Iron content in Aluminum is 0.25% MAX (upper spec limit). Our data averages 0.10%.[/font]

[font=Times New (W1)]Here is my understanding (please correct me)[/font]

[font=Times New (W1)]This software seems to assign 0 as the lower spec limit. This is hard coded, no option to change it.[/font]

[font=Times New (W1)]Process capability (Cp) should not be determined since there is no tolerance as such.[/font]

[font=Times New (W1)]However, the software can mathematically determine Cp by using 0 to 0.25% as the Tolerance.[/font]

[font=Times New (W1)]I believe Cp has little meaning in the case of unilateral tolerances – am I right?[/font]

[font=Times New (W1)]A better measure would be the Taguchi loss function, since it equates loss(cost) to how far from (close to) ideal you are.[/font]

[font=Times New (W1)]Process Capability Centered (Cpk) is even worse.[/font]

[font=Times New (W1)]The software will take the lesser of AVG- LSL or USL-AVG and produce a “mathematically correct” Cpk value.[/font]

[font=Times New (W1)]I believe this is wrong since it results in a poor Cpk, since our process averages much closer to 0 than to the USL. [/font]

[font=Times New (W1)]I believe the Cpk calculation should only be determined for USL in the case of a unilateral tolerance – am I correct?[/font]

[font=Times New (W1)]Interestingly we also have MiniTab, and it seems to do things correctly (if you set it up right).[/font]

[font=Times New (W1)]Minitab will calculate a Cp if you enter 0 as the lower spec limit. However, if you leave the LSL blank, Minitab will not report a Cp value. This makes sense to me.[/font]

[font=Times New (W1)]Also Minitab will calculate Cpk from the USL if you leave the LSL blank. However, if you enter 0 it will report the lower value.[/font]
[font=Times New (W1)][/font]

[font=Times New (W1)]Looking forward to an intense learning experience[/font]

[font=Times New (W1)][/font]
[font=Times New (W1)]Caster[/font]






 
Elsmar Forum Sponsor

Paul F. Jackson

Quite Involved in Discussions
#2
Check the distribution for normality, use Minitab, and leave the LSL blank like you said or get the fix for the other software. I think that you have already found your own solutions to the problem.
 
D

Darius

#3
Caster :agree1:

If your software takes 0 as the other specification buy another one. It's like sometime ago I told someone, it's like using a table without all the background or objective of it. If Cpk is calculaded by the software the way you said it's even worse. :bonk:

I like to see someone that likes Taguchi Index as an indicator, the Cpmk could help more than the cpk for one spec conditions. Why, because Cpk is as you said CENTERED.

You said
I believe the Cpk calculation should only be determined for USL in the case of a unilateral tolerance – am I correct?
if you only have one spec you at least do it with the only one that you have, but IMO it's as you said better to use Cpmk. Or nonparametrical if your data behaves non gausian ("normal").
 
A

Arvind

#5
Dealing with non-normal data and one sided specification limits

Many types of data, for example- flatness, runout, impurities or material composition which has one sided limits is observed to be non normal. The reason for non normality in most cases is likely to be
a) Outliers
b) bimodal or multimodal data
In such cases, normality assumption and calculation of capability can give errors which may not be desirable.

It may be desirable to fit Weibull distribution and find shape and scale factors (which define Weibull) from the data and find how much of tail portion of distribution exceeds the threshold limit.
Many times fraction of population exceeding specification is of importance than capability by itself.

Arvind
 
Thread starter Similar threads Forum Replies Date
D Siemens MES software DPMO Calculation Six Sigma 4
B Attribute MSA calculation - Trying to find a form or software Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 4
Q Software as a medical device vs software not sold as medical device: local regulations for sale? EU Medical Device Regulations 4
Y Software updates considered servicing (7.5.4) ISO 13485:2016 - Medical Device Quality Management Systems 4
S How to perform verification of the Statistical Analysis Software? Qualification and Validation (including 21 CFR Part 11) 2
I Document Control Software Document Control Systems, Procedures, Forms and Templates 2
E Software maintenance Process Software maintenance Process to IEC 6204? IEC 62304 - Medical Device Software Life Cycle Processes 3
L Micro-Vu InSpec Software Program Qualification and Validation (including 21 CFR Part 11) 6
A For software change - New Channel of interoperability CE Marking (Conformité Européene) / CB Scheme 4
T IVDR Medical device software CE Marking (Conformité Européene) / CB Scheme 8
N ISO 13485 7.3.9 Change control in medical device software ISO 13485:2016 - Medical Device Quality Management Systems 6
C SharePoint Contract Management Software General Information Resources 0
gramps What do you think about automated QA testing For software app industry? Misc. Quality Assurance and Business Systems Related Topics 5
V Software as medical device (SaMD) replicated for multiple clients through APIs IEC 62304 - Medical Device Software Life Cycle Processes 5
U API Spec Q1 - 5.6.1.2 C (3) - Design software Oil and Gas Industry Standards and Regulations 3
B Complaint Records - Accessing records on Easy Track Software Records and Data - Quality, Legal and Other Evidence 3
GreatNate Master Control QMS software Quality Tools, Improvement and Analysis 0
GreatNate Anyone using the Intellect QMS software? Quality Assurance and Compliance Software Tools and Solutions 1
S DHF/DMR/MDF for a software-only, cloud-based, single-instance device Medical Information Technology, Medical Software and Health Informatics 2
H Software Validation for FFS Packaging Machine Qualification and Validation (including 21 CFR Part 11) 1
E Any sample of a full software life cycle IEC 62304 report ( any class )? IEC 62304 - Medical Device Software Life Cycle Processes 1
Q ISO 13485 7.5.6 Validation - Off the shelf Software ISO 13485:2016 - Medical Device Quality Management Systems 3
M ERP / QMS related software standards for Validation IEC 62304 - Medical Device Software Life Cycle Processes 6
J Do Software Subcontractors need to be ISO13485 compliant in the EU? EU Medical Device Regulations 3
D Safety data sheets software REACH and RoHS Conversations 2
N What are the software audit and control steps Reliability Analysis - Predictions, Testing and Standards 2
N Validating Software before getting approved as Class 2 device US Food and Drug Administration (FDA) 5
M Clinical Decision Support Software Question 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 2
P Missing 1m visual alarm signal in case of software/display failure, mitigation? ISO 14971 - Medical Device Risk Management 3
B Software service provider as critical supplier ISO 13485:2016 - Medical Device Quality Management Systems 5
S Asterisk in DOE minitab software Using Minitab Software 23
M Surgical angle measurement guide device with an application software Medical Device and FDA Regulations and Standards News 1
M Advice needed for SEH Compliance Software and ISNETWord Compatabiliy Occupational Health & Safety Management Standards 2
bruceian Software Quality Metrics Software Quality Assurance 11
optomist1 How Secure Are Our Software Systems Software Quality Assurance 7
M 'Active' device? Software/laptop with attached camera 'looking' at passive metal probe EU Medical Device Regulations 3
D Software validation team Misc. Quality Assurance and Business Systems Related Topics 3
O Any info on release date of FDA “Computer Software Assurance for Manufacturing and Quality System Software” document? 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 0
L Radiology software Class I exemption Medical Device and FDA Regulations and Standards News 3
O Software for comparing text of PDF files Contract Review Process 2
J Implementing an ISO 13485 QMS Software ISO 13485:2016 - Medical Device Quality Management Systems 6
K Software Updates in the Field and ISO scope ISO 13485:2016 - Medical Device Quality Management Systems 2
M Recurrent event analysis software (python) General Auditing Discussions 2
Y UL 1998 Standard: software classes Software Quality Assurance 0
P Need a programmer for QVI's VMS software for optical inspection machine Inspection, Prints (Drawings), Testing, Sampling and Related Topics 0
S IEC 62304 software costs and time Medical Device and FDA Regulations and Standards News 3
S IEC 62304 - Software verification cost IEC 62304 - Medical Device Software Life Cycle Processes 3
Sravan Manchikanti Software Risk Management & probability of occurrence as per IEC 62304 IEC 62304 - Medical Device Software Life Cycle Processes 8
I Form templates for software (iso9001) Document Control Systems, Procedures, Forms and Templates 0
H Software Interface Translation IVD Regulation EU Medical Device Regulations 0

Similar threads

Top Bottom