Statistically Valid Sample Sizes For SPC

J

J. Otto

#1
Hello All,

I am looking for some input into sample size considerations and frequecy of sample pulls for production lots that we are calculating Ppk values on. Currently we are pulling 4 parts every 2 hours for each process to calculate Ppk values on. My concern is that this is the 4 parts every 2 hours is regardless of cycle time or production lot size.

Is there an issue with being statistically valid in our calculations based on sample size and frequency. As an example, I may have two seperate production lots that I am going to run for 24 hours. The first process may have a cycle time of 10 seconds and the second process may have a cycle time of 20 seconds. After 24 hours of production, this would result in first process having twice as much product than the second process. Should we be increasing our sample size and/or frequency of pulls for the first process?????

If there is any information available or if anyone has any suggestions on what makes for statistically valid sample sizes and frequency of sampling, please let me know.

Any help will be greatly appreciated.

Regards,

Jon
 
Elsmar Forum Sponsor
R

Rick Goodson

#2
Your question is very good, but unfortunately complex and not easily answered in a paragraph or two. Never the less, some thoughts and suggestions.

From a protection point of view, absolute sample size of a random sample is more important than its relative size compared to lot size. This is readily apparent if you look at Operating Characteristic Curves for a constant sample size and with differing lot sizes.

With regard to sample size in X-bar and R charts, the essential idea is to select subgroups to minimize the opprotunity for variation with-in a subgroup. We want them small. However, a size of four works best because with a sample of four the distribution of X-bar is nearly normal even though the samples are taken from a non-normal universe.

The frequency of the sampling is an economic decision based on what happens to material produced between samples if we find an out of control condition. Theoretically frequency should be often when analyzing a process initially and the reduced once the process is under control and running well.

Take a look at the following references for detail on sample size selection and frequency:

Statistical Process Control by Grant and Leavenworth, ISBN 0-07-844354-7

AT&T Statistical Quality Control Hanbook (Mine is so old id does not have an ISBN)

Quality Control by Dale Besterfield
ISBN 0-13-025668-4
 
Thread starter Similar threads Forum Replies Date
F Sample Size to get Statistically Valid Data Measurement Uncertainty (MU) 6
K Statistically Valid Audit Sample for Management System Audits? Inspection, Prints (Drawings), Testing, Sampling and Related Topics 4
S Statistically valid number of biopsy devices should be tested US Food and Drug Administration (FDA) 4
P Sterile Packaging Validation - FDA Requirements for Statistically Valid Tests 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 9
S Statistically valid method for multiple raw material sampling Inspection, Prints (Drawings), Testing, Sampling and Related Topics 4
H How to Select a Statistically Valid Sampling Plan Inspection, Prints (Drawings), Testing, Sampling and Related Topics 10
S How to Statistically Prove 2 Dimensions are Related Using Minitab Software 18
D Best Minitab Tool to determine a Statistically Significant Trend Using Minitab Software 5
F How to statistically determine the maximum performance life of a chemical Statistical Analysis Tools, Techniques and SPC 4
H Process that is Not Stable vs. Not Statistically Capable - Differences Capability, Accuracy and Stability - Processes, Machines, etc. 9
J Demonstrate Receiving Inspection Sampling Method is Statistically Adequate Inspection, Prints (Drawings), Testing, Sampling and Related Topics 3
N Statistically Comparing Technician Techniques Reliability Analysis - Predictions, Testing and Standards 16
B Statistically Sound Data Sampling Technique for Internal Audits Inspection, Prints (Drawings), Testing, Sampling and Related Topics 14
S Getting started with statistically based process improvement The Reading Room 12
M Statistically Toleranced Drawing Feature Dimensions - Target Value and Range Statistical Analysis Tools, Techniques and SPC 5
L How to Monitor Corrective Actions - Statistically Monitoring Corrective Action Nonconformance and Corrective Action 34
G Pp and Ppk indices - Is it statistically possible to have a Ppk greater than Pp? Capability, Accuracy and Stability - Processes, Machines, etc. 16
M Auditing Statistically - A statistical approach to sample sizes General Auditing Discussions 11
J Statistically Determining how many Tests to conduct. Statistical Analysis Tools, Techniques and SPC 3
B IATF 16949 News Six month extension on all valid IATF 16949 certs IATF 16949 - Automotive Quality Systems Standard 10
H PCD remains valid for new product ISO 13485:2016 - Medical Device Quality Management Systems 11
C Neglect or legitimate deferral? Is excessive workload or lack of resources in a department or a team a valid root cause for a non-conformance? Problem Solving, Root Cause Fault and Failure Analysis 12
M Products Manufactured while CE was valid CE Marking (Conformité Européene) / CB Scheme 1
L Valid reasons to omit an "ear-tag" type label on a cable? IEC 60601 - Medical Electrical Equipment Safety Standards Series 1
J Are Capability Requirement(s) Valid for All Characteristics? Capability, Accuracy and Stability - Processes, Machines, etc. 6
M Are ASTM F745 & ASTM F75 still valid? EU Medical Device Regulations 5
Q Is KPI in Process Valid if not supported by Procedures? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 5
K MDD or other EU guidance that describes the need for Valid Production Processes EU Medical Device Regulations 2
T Predicate device recalled - Still a valid predicate device? Other US Medical Device Regulations 7
M Is calibration from long ago still valid on never used pipettes? General Measurement Device and Calibration Topics 8
M FDA 21 CFR 820.250 - Does "valid statistical" always mean math? 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 6
C Is this GR&R method valid? (Printing Company) Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 4
A CE Certificate is valid until 3.12.2013 - Can I still sell the products in Europe? EU Medical Device Regulations 10
somashekar Authorization to use shelf life sensitive materials beyond its valid shelf life... Manufacturing and Related Processes 7
V Sample Range for a valid GRR using ANOVA in Minitab Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 2
Q Is it Valid to refer group of documents instead of names? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 9
G Gage R&R Calculation Spreadsheets - Which one here is valid? Quality Tools, Improvement and Analysis 4
K Who issued this certificate and what products it is valid for? ISO 13485:2016 - Medical Device Quality Management Systems 5
H ISO 9001:2000 Certificate - Original approval date 18 July 1995 and valid until 2012 ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 27
W How long is a Medical Device product registration in Iran valid for? Other Medical Device Regulations World-Wide 1
R Is Cpk valid for a sample size of "1"? Capability, Accuracy and Stability - Processes, Machines, etc. 5
S ISO/TS 16949 certification no longer valid when automotive part production ends? IATF 16949 - Automotive Quality Systems Standard 7
P Valid NB Technical Review Nonconformance? New Class IIb Product EU Medical Device Regulations 4
B Is ISO 14971:2007 still a valid standard? ISO 14971 - Medical Device Risk Management 9
S Without Surveillance Audit, Is ISO certification still valid? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 8
R Is Cpk calculation of dBA (noise level) valid? Capability, Accuracy and Stability - Processes, Machines, etc. 9
Q How long is ISO 9001:2000 good for (valid) ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 2
Q Is my Cpk calculator answer valid? Capability, Accuracy and Stability - Processes, Machines, etc. 4
6 Why the general Valid period of QMS certificate is 3 years? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 8
M Do we need to cite applicable clause of ISO9001 std for a valid customer complaint Customer Complaints 8

Similar threads

Top Bottom