SBS - the best value in QMS software

How to decide on a sampling size for Xbar-R charts

B

bmunger

#1
Hello,

I've found one discussion on the internet of this subject here (the link is intentionally crippled because of this forum's restrictions, you need to fix it by removing the spaces around the domain name):
www . winspc . com /what-is-spc/ask-the-expert/334-how-should-the-subgroup-size-be-selected-for-an-x-bar-chart-part-i

But the formulae presented in textbooks vary a lot and I'm unsure how to choose the right formulae to use. There are 8 different forumlae presented in the textbook from Wiley entitled "Probability and Statistics in Engineering and Management Science" among which the one from the internet reference above is present.

But it's not presented as THE one that should be used, in fact none of them are.

So how to we choose which formulae to use to determine the sample size? Is that something personal or is there a scientific/mathematical basis for it?

Thanks!
Bernard
 
Elsmar Forum Sponsor

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#2
Generally the sampling size should be "rational", based upon the process itself. Such as perhaps a "production lot" happens to be 12 items. If pushed for a number, I've generally seen "4 or 5", with 4 having historical preference because it is easy to figure out the square root of four.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#3
To add to Steve's response, the sample size is not generally 'statistically' selected - except potentially for categorical - or pass/fail - data. All of the common sample size formulas are meant for 'point estimates' of some population or static set of data. Control Charts are for monitoring processes over time. This enables the use of much smaller sample sizes. As Steve alluded to the size typically is based in some rational understanding of how the process can vary. Typically 3-5 sequential pieces is sufficient for moderate to high volume production. For lower volume - such as with functions of complex assemblies I use a subgroup size of 1 and measure every assembly.

The best way is to plot your data in time series and then think about the science of the process and the data - what makes sense? how many samples do you need for each subgroup and how often should you subgroup in order to detect changes?
 
Thread starter Similar threads Forum Replies Date
H How to decide AQL value? Sampling Plans AQL - Acceptable Quality Level 5
silentmonkey How to decide what characteristics need to be verified during incoming inspection? ISO 13485:2016 - Medical Device Quality Management Systems 5
S How to decide which medical device symbols go on label and labeling? US Food and Drug Administration (FDA) 4
J ISO 9001 - How to decide if a company needs or will benefit from certification? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 6
L Help me to decide which test to use - If each proportion belongs to the same population Using Minitab Software 7
T Can we remove a quality objective if we decide it is not measurable? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 11
J How to decide work-stream in scope of Quality - Roles and Responsibilities ISO 13485:2016 - Medical Device Quality Management Systems 1
D Service Identification - How can we decide if X is a service or not? Lean in Manufacturing and Service Industries 4
R How to decide a suitable contact time from table 23 of IEC 60601-1:2005 IEC 60601 - Medical Electrical Equipment Safety Standards Series 6
C Medical Device Manufacturing in Controlled Environment? How to decide? Other Medical Device Regulations World-Wide 1
M Statistical Method to decide Threshold Value from a Data Set Statistical Analysis Tools, Techniques and SPC 1
P Gauge Design Tolerances - How to decide tolerances for gauge design? General Measurement Device and Calibration Topics 3
C Starting an MSA study - How to decide which is appropriate or required Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 2
S Lean or Six Sigma or TOC (Theory of Constraints) - How to decide Lean in Manufacturing and Service Industries 6
A Who to decide and select Internal Auditors and what are the criteria? Internal Auditing 18
M CAPA Rankings to decide if action is required Nonconformance and Corrective Action 7
D Who should decide on my salary and adjustments? Career and Occupation Discussions 9
P How do you decide what is an acceptable Internal and External PPM number? IATF 16949 - Automotive Quality Systems Standard 17
M Overhaul QMS or start over? How do you decide? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 40
P How do you decide what is a Process, a Procedure or Work Instruction? Document Control Systems, Procedures, Forms and Templates 89
J To Decide Which Factor has the Highest Influence on the Results Using Minitab Software 3
K Regulatory Requirements for ISO 13485 - Which country to decide? ISO 13485:2016 - Medical Device Quality Management Systems 6
B How to decide the ?U? to calcuate DPU and "opportunity" of DPMO Six Sigma 8
L Is the EPA is going to decide if the SPCC deadline is to be extended? Miscellaneous Environmental Standards and EMS Related Discussions 1
M CA (corrective action) or Continuous Improvement? It's hard to decide. Nonconformance and Corrective Action 19
M how to decide the submission level for PPAP APQP and PPAP 2
Y How do we use SPC or other stat tools to decide product spec? Statistical Analysis Tools, Techniques and SPC 5
D On-going capability - Can we decide for ourselves how often to measure cpk? Capability, Accuracy and Stability - Processes, Machines, etc. 6
M Minitab Capability of the Population (no sampling) Using Minitab Software 11
W LTPD, AQL, Ppk and Cpk validation sampling plan table Inspection, Prints (Drawings), Testing, Sampling and Related Topics 0
Q Sampling method for Distributor verification activities EU Medical Device Regulations 1
E Change in control plan - Do I have to do sampling? IATF 16949 - Automotive Quality Systems Standard 1
S Is using ANSI/ASQ Z1.4-2008 the correct sampling plan to determine Pass/Fail of Apparel measurements? AQL - Acceptable Quality Level 4
N Sampling Plan for Internal Audits - ISO 2859 or 3951 - Or Neither? Internal Auditing 6
F Product audit sampling plans IATF 16949 - Automotive Quality Systems Standard 3
R Reduced sampling plan for sterial products APQP and PPAP 0
A ISO 2859-1 - 9.3.3.2 Switching score - Sampling Plans and PPM AQL - Acceptable Quality Level 0
R BS EN 12060:1998 - Zinc and zinc alloys Method of sampling Specifications Manufacturing and Related Processes 4
S High voltage testing - ISO 17025 - 7.2.2 Validation of methods and 7.3 Sampling ISO 17025 related Discussions 3
8 Activity Sampling Exercise Lean in Manufacturing and Service Industries 11
T Defining sampling plan for different AQL AQL - Acceptable Quality Level 3
B AS9100D Sampling Plan AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 2
R Design verification for interim design outputs - sampling rationale ISO 13485:2016 - Medical Device Quality Management Systems 2
Nicole Desouza Sampling plan for a customer who wants AQL 1.0 (per ANSI Z1.4) AQL - Acceptable Quality Level 5
V How to interpret AQL sampling tables AQL - Acceptable Quality Level 5
A AQL sampling bags of parts Inspection, Prints (Drawings), Testing, Sampling and Related Topics 5
C Applying Weibayes to Attribute sampling Reliability Analysis - Predictions, Testing and Standards 0
I Sampling processes - Who must define the AQL level? AQL - Acceptable Quality Level 9
E Sampling for capability studies for variables and attributes Capability, Accuracy and Stability - Processes, Machines, etc. 4
O Statistical justification of sampling size in V&V tests ISO 13485:2016 - Medical Device Quality Management Systems 5

Similar threads

Top Bottom