How to deal with nonrandom samples?

K

KenK - 2009

What do you want to do with them?

Most common statistical methods assume that data have been aquired randomly from their population. If they are not, a managerie of possible problems can emerge.

J

jay

I have a situation where sampling was selected by rank ordering the hardware for a characteristics perceived to be the influencing variable and testing only n worst samples based on 105. The assumption was that at 1% AQL 0 failures on the test would depict acceptance of the lot. Of course, the test failed. Now the situation is to accept the lot at a higher AQL (<4%) or do additional testing. How many more to test for zero failures so that the lot can be certified to be 99% good with some (95%) confidence. Because of the bias, I tests show no correlation of test to assumed influencing variable indicating the presence of other factors. The idea is to avoid 100% inspection which may be the way to go with the high degree of confidence required. Any ideas?

R

Rick Goodson

jay,

I may not understand the situation, but it appears that you have decided on a test to determine whether a lot of material is acceptable for shipment. The lot has failed so you now want to change the parameters of the test so that the lot will pass. If that is the case, it is relatively simple to determine what the sampling size and acceptance number should be to pass the lot. IMHO however, you should decide whether or not this is really what you want to do.

If you want a sampling plan that provides 95% confidence that the lot of material is no worse than 1% 'defective' you can use a standard sampling scheme or design a custom sampling scheme to provide that protection. Either will require you test a number of samples randomly selected from the lot. If you want to estimate the the number of defectives in a lot of material at some confidence level, you can also do that. Similarly it will required a number of random samples selected from the lot. Any good text on statistics/sampling can help you with either of the options I mentioned.

J

jay

Rick:
Yes, your observations are correct to the extent that the acceptance parameters can be changed to pass a test. I can go from an AQL of 1% to 4% under 105 and pass the lot. There will be several issues in using such approaches, least of which is a cloud on already passed lots. Further analysis showed that the lack of correlation and covariance is due to the early samples under going additional influencing variables. When these were removed from the analysis, the rest tracked what we perceived to be a better correlation. I am more interested in learning how the statisticians would handle non-random situations. The approach I intend to use is to increase the sample size dropping the early non-conforming items and redo the math. Further testing will show if our influencing variable is a dependable measure. I did not see any mathematical techniques in the books I looked at or on the web. I have been researching this over past three days. Thanks for your input.
Jay

How to deal with an ISO 13485 Supplier Audit nonconformance ISO 13485:2016 - Medical Device Quality Management Systems 17
How to deal with FDA after not reporting anything for several years Other US Medical Device Regulations 14
Informational Update from GOV.UK – Regulating medical devices in the event of a no-deal Brexit – UK Responsible Person Medical Device and FDA Regulations and Standards News 0
How to deal with changed shared components in 510k 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 2
Informational UK – Contingency legislation covering regulation of medicines and medical devices in a no deal scenario – Human Medicines and Medical Devices (Amendm Medical Device and FDA Regulations and Standards News 3
Informational EU – Medicinal products and medical devices: Coordinated approach in case of a withdrawal of the United Kingdom from the Union without a deal Medical Device and FDA Regulations and Standards News 0
Informational UK – Regulating medical devices in the event of a no deal scenario Medical Device and FDA Regulations and Standards News 0
Informational UK – Businesses supplying medicines and medical devices – what to expect on day one of a ‘no deal’ scenario Medical Device and FDA Regulations and Standards News 1
Informational Design for new product safety marking for the no-deal Brexit scenario Medical Device and FDA Regulations and Standards News 1
Informational UK – Contingency legislation covering regulation of medicines and medical devices in a no deal scenario Medical Device and FDA Regulations and Standards News 1
How to deal with catalog parts suppliers who refuse to submit PPAP documents? APQP and PPAP 0
Medical Device News MHRA releases response to consultation on EU exit no-deal legislative proposals Medical Device and FDA Regulations and Standards News 0
Pharmaceuticals News UK – Further guidance note on the regulation of medicines, medical devices and clinical trials if there’s no Brexit deal Medical Device and FDA Regulations and Standards News 0
Medical Device News Letter to the health and care sector: update on preparations for a potential no-deal Brexit Medical Device and FDA Regulations and Standards News 0
Indemnity Letters - What's the Deal? Other Medical Device and Orthopedic Related Topics 5
Medical Device News MHRA to consult on EU exit no-deal legislative proposals EU Medical Device Regulations 1
How to deal with user needs when it is obvious the design meets the user need 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 5
P Quality Assurance and Quality Control - Which clauses of ISO 9001 deal with each? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 3
How to deal with Multiple Datums - Position with respect to Multiple Datum Feature Inspection, Prints (Drawings), Testing, Sampling and Related Topics 7
ISO 9001:2015 Clause10.2 Nonconformity and Corrective Action - Deal with Consequences ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 2
How to deal with resistance to GDP Document Control Discipline Document Control Systems, Procedures, Forms and Templates 7
R How to deal with CONFIDENTIAL documents Document Control Systems, Procedures, Forms and Templates 2
Interesting Discussion How to Deal with Suppliers Who Refuse to Complete our ISO Survey? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 51
L How to deal with resistance from auditee(s) Internal Auditing 20
How to deal with a Recruitment Consultant Career and Occupation Discussions 3
3 Ways To Deal With People Who Play Office Politics Against You Coffee Break and Water Cooler Discussions 7
J How to deal with incomplete forms Nonconformance and Corrective Action 14
How to deal with the RM requirement of clause 17 Electromagnetic Compatibility? IEC 60601 - Medical Electrical Equipment Safety Standards Series 8
How to deal with too many CARs (Corrective Action Requests), PARs (Preventive Action) Nonconformance and Corrective Action 25
T How to train employees to deal with external auditors? Internal Auditing 13
S Training on how to deal with Regulations, Rules, Regulations Conflicts, etc. Training - Internal, External, Online and Distance Learning 6
M Mobile Medical Platforms - How to deal with Supplier Hardware/Software Changes? 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 2
S How to deal with non-compliant company after take over of them IEC 27001 - Information Security Management Systems (ISMS) 6
C If my location does not deal with GM - Firewall - CS1 & CS2 Question IATF 16949 - Automotive Quality Systems Standard 3
I How to deal with Behaviour Problem in the work place? Human Factors and Ergonomics in Engineering 20
J How to deal with Mental models IEC 62366 - Medical Device Usability Engineering 1
Deal? Or no Deal? Coffee Break and Water Cooler Discussions 25
Looking for a good deal on a boat? World News 2
How to deal with telemarketers Funny Stuff - Jokes and Humour 2
Giving training with a vicious cold - how do you deal? Training - Internal, External, Online and Distance Learning 9
Delphi to close, sell most plants under deal World News 0
How to deal with an employee who has a bad attitude? - Employee Attitudes Coffee Break and Water Cooler Discussions 15
D X Bar Chart - How can I deal with missing values in subgroups Statistical Analysis Tools, Techniques and SPC 12
E Lets Make a Deal... The TV show - A statistical approach Coffee Break and Water Cooler Discussions 8
Dock to Stock - How to deal with inspection status requirements Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1