What frequency and sample size is 'normal' for SPC?

N

narfeldt

#1
Hello,

I have a detect a critical characteristics in our control plan and I want to control it with SPC.
Which frequency is normal for SPC?

The same problem I've got for MSA and capability study!

Thanks for some help

[This message has been edited by narfeldt (edited 03 July 2001).]
 
Elsmar Forum Sponsor
A

Al Dyer

#2
I think we need some more detailed information to give a good response.

Basically start off with a sample size of 5 with a high frequency rate until capability is established.

MSA can be a whole different animal, need more detail.

ASD...
 
R

Rick Goodson

#3
Narfeldt,

This topic should bring some interesting comments.

From the Automotive Industry Action Group publication on Statistical Process Control page 31. "Subgroups should be collected often enough, and at appropriate times, that they can reflect the potential opportunities for change...Subgroup frequencies for ongoing production monitoring could be twice per shift, hourly, or some other feasible rate." Interesting, but not very useful.

From Statistical Quality Control by Grant and Leavenworth, page 42. "No general rules may be laid down for frequency of subgroups. Each case must be decided on its own merits, considering both the cost of taking and analyzing measurements, and the benefits to be derived from the action based on the control charts." Better, but still not much help.

In Dale Besterfield's book Quality Control he suggests that it is a cost-benefit analysis but if you absolutely need a rule of thumb use the Normal Inspection, Level IV sample size table from the old US military standard MIL-STD-414. As an example, if you produce 4000 pieces a day you should look at 60 pieces. With a subgroup size of four that would be 15 subgroups or about one every 30 minutes across an eight hour shift. If the subgroup size is 5 you would have 12 samples or one every 40 minutes.

Regards,

Rick
 
C

Cristi?nC

#4
Originally posted by Rick Goodson:

Narfeldt,

This topic should bring some interesting comments.

From the Automotive Industry Action Group publication on Statistical Process Control page 31. "Subgroups should be collected often enough, and at appropriate times, that they can reflect the potential opportunities for change...Subgroup frequencies for ongoing production monitoring could be twice per shift, hourly, or some other feasible rate." Interesting, but not very useful.

From Statistical Quality Control by Grant and Leavenworth, page 42. "No general rules may be laid down for frequency of subgroups. Each case must be decided on its own merits, considering both the cost of taking and analyzing measurements, and the benefits to be derived from the action based on the control charts." Better, but still not much help.
Please let me rise some other questions.

Definitely, there is no rules for a topic like "sampling".

I suggest you to take a "progressive" approach in order to reach some optimal sampling plan. Technically:

1) Make a general view of the process through a "pre-sample". Don't worry with sample sizes and frequencies because you are not making decisions that will affect the process right now. Be careful to take a good sample that represents the majority of the conditions that rules the process.

2)Calculate the Standard Deviation of that sample.

3) Use any software that allows sample size calculations (e.g. Minitab). You must inform the "power" and the "confidence level" for that. Both of them refers to the probabilities of making wrong decisions. Declare several "high values" for them (minimum 0.7 for power and 0.9 for CL) The software will list a series of sample sizes each one associated with the probability of making wrong decisions.

4)Choose the better one that fits your budget and time restrictions (unfortunately all these calculations do not take into account such "terrenal" stuff!)

5)Start de SPC with that sample. With this sample you WILL make decisions. A bit of time will tell you if it is ok. If no, at least you will have a better estimate of the standard deviation to re-start these steps!

Is it too complicated?
 
G

Graeme

#5
Originally posted by narfeldt:
Which frequency is normal for SPC?
The same problem I've got for MSA and capability study!
A short and sassy answer is, "how much bad product can you afford to ship?" :D

Really, as other members have suggested, it is difficult to say how often to take samples if you are just starting. I agree with Rick's suggestion of using MIL-STD-414 as a guide. It has been replaced by ANSI/ASQC Z1.4-1993, and can be purchased in electronic or print form from ASQ ( www.asq.org ). Once you start making measurements, examine the data to understand what it is telling you. However, be careful not to fall into the twin traps of too many samples or too few samples - either one can lead you astray.

Graeme
 
Thread starter Similar threads Forum Replies Date
P Frequency and Sample Size Requirements for an MSA Studies Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 4
B Customer Inspection Frequency and Rejection Level Criteria - Sample Lot Size AQL - Acceptable Quality Level 6
L Criteria for Establishing Control Plan Sample Size and Inspection Frequency FMEA and Control Plans 7
Manix GM's GP12 Increased Frequency/Sample Size - B1 of the plan development section FMEA and Control Plans 8
A Sample Size and Frequency - In-process monitoring in a mass production shop Statistical Analysis Tools, Techniques and SPC 2
G Determining Sample Size and Frequency for Features on Control Plans FMEA and Control Plans 8
B Sampling Plan to determine sample size/frequency based on a Given Rate of Production Inspection, Prints (Drawings), Testing, Sampling and Related Topics 40
R How to determine the correct Sample Frequency for SPC Features Statistical Analysis Tools, Techniques and SPC 66
R Procedure, Frequency and Acceptance Criteria for Replicate, Recalibration, Before-After and Intermediate Checks ISO 17025 related Discussions 1
Q To set frequency to review documents in ISO 9001 7.5? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 10
M Defining frequency of measurement tools callibration Calibration and Metrology Software and Hardware 3
M Define voltage and frequency to perform tests 61010-1 and 61326-1 for CE certification CE Marking (Conformité Européene) / CB Scheme 4
L Calibration Frequency for Surftest SJ-400 General Measurement Device and Calibration Topics 0
D Determining Calibration Frequency schedule for items used in production Manufacturing and Related Processes 2
G ISO 9001 - 9.3.1 Management Review - Content and Frequency Management Review Meetings and related Processes 12
H Frequency of shelf life/sterilization/bench studies conducted on a medical device Other Medical Device Related Standards 1
M RFID (Radio Frequency Identification) Registration in Europe and in MENA countries EU Medical Device Regulations 1
M Leakage current measurement with non-frequency-weighted device IEC 60601 - Medical Electrical Equipment Safety Standards Series 1
S Requirements for Interval Measurement test & Frequency Response test clarification IEC 60601 - Medical Electrical Equipment Safety Standards Series 0
B Radiation Sterilization Dose Audit: Bioburden frequency Other Medical Device Related Standards 1
bobdoering SPC Charting frequency calculator for X hi/lo chart Imported Legacy Blogs 0
D IATF 16949 surveillance audit frequency IATF 16949 - Automotive Quality Systems Standard 16
C AS9100 rev D 8.5.1 c 2 - Defining the Machine in-process frequency per ANSI/ASQ Inspection, Prints (Drawings), Testing, Sampling and Related Topics 8
J 95% reliability for calibration frequency Calibration Frequency (Interval) 7
R Internal Audit Frequency - Remote Site (a Design Center) IATF 16949 - Automotive Quality Systems Standard 3
V Periodic Audit Trail Review - Scope, Content & Frequency Qualification and Validation (including 21 CFR Part 11) 10
JoCam Required Frequency of Notified Body reviews of Technical Files EU Medical Device Regulations 6
J AMS 2750E System Accuracy Test (SAT) Frequency AS9100, IAQG 9100, Nadcap and related Aerospace Standards and Requirements 3
B MHRA Unannounced Audits Frequency Changed? EU Medical Device Regulations 2
P IATF 16949 - Internal Audit Frequency Requirements Internal Auditing 2
Q Objective and Goals - ISO 9001:2008 Frequency Revision Requirements ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 1
Q RFID (radio frequency identification) registration for Medical Device Other Medical Device Regulations World-Wide 6
G Sequence and Frequency of Calibration for Thread Gauges General Measurement Device and Calibration Topics 2
N Tool frequency change - Data Analysis needed Statistical Analysis Tools, Techniques and SPC 9
GStough Adequately Defining Which Suppliers to Audit and Frequency Supplier Quality Assurance and other Supplier Issues 8
Y FDA's Inspection Frequency for Foreign Manufacturer Other US Medical Device Regulations 1
R TS 16949 Process Re-Validation Frequency Requirements IATF 16949 - Automotive Quality Systems Standard 3
Marc Calibration Frequency (Interval) Discussions Sub-Forum General Measurement Device and Calibration Topics 0
kedarg6500 Could I have a recalibration frequency of +/- 15 days ? Calibration Frequency (Interval) 23
J Appropriate In-Process Monitoring Frequency for 2X per day Change Set-Up. Statistical Analysis Tools, Techniques and SPC 3
S Recertification Frequency for TS 16949 Lead Auditors General Auditing Discussions 4
Gman2 Frequency of Internal Audits per TS 16949 Requirements IATF 16949 - Automotive Quality Systems Standard 12
J Digital Scale Re-Calibration Interval Frequency General Measurement Device and Calibration Topics 10
M A Guide or Standard for Calibration Interval (Frequency) Calibration Frequency (Interval) 2
P Frequency of Bioburden Testing EU Medical Device Regulations 3
smryan Weight Calibration Standards - External Calibration Frequency General Measurement Device and Calibration Topics 11
G UID Compliance Verifier Calibration Test Card Calibration Frequency General Measurement Device and Calibration Topics 3
R Why is 10 MHz cited as the reference frequency? Measurement Uncertainty (MU) 2
M Decrease frequency of inspection based on results Inspection, Prints (Drawings), Testing, Sampling and Related Topics 3
S How to incorporate a customers special inspection frequency in our control plan FMEA and Control Plans 1
Similar threads


















































Top Bottom