AQL (Acceptable Quality Level) for Zero Defects

Elsmar Forum Sponsor

Bev D

Heretical Statistician
Staff member
Super Moderator
#12
Re: AQL for Zero Defects

If it is not homogeneous, then it has to be random and large enough to capture the variation. If not homogeneous - may require a larger sample than the standard plans to be effective. Lot definition can be a contributor to the problem (e.g. several streams combined at a batch process that become a lot may be an issue to detect its true variation with insufficient sampling.)

Sampling is so fraught with error that if it is not homogeneous and normally distributed you are likely to get unreliable results.
Acceptance (random) sampling from a completed lot is not the same as SPC where a non-homogeneous process stream is sampled in a stratified manner in time sequence. The statistics are different (and not just theoretically but in reality). Random sampling from a non homogeneous lot actually 'homogenizes' the distribution of results within the sample. AND for categorical sampling the underlying distribution is irrelevant as the number of defects in the sample is a function of probability theory not distributional statistics. The 'normality' of the distribution - in this case - is irrelevant.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#13
Re: AQL for Zero Defects

Acceptance (random) sampling from a completed lot is not the same as SPC where a non-homogeneous process stream is sampled in a stratified manner in time sequence. The statistics are different (and not just theoretically but in reality). Random sampling from a non homogeneous lot actually 'homogenizes' the distribution of results within the sample. AND for categorical sampling the underlying distribution is irrelevant as the number of defects in the sample is a function of probability theory not distributional statistics. The 'normality' of the distribution - in this case - is irrelevant.
Point well taken. It is good to bring up what the statistical requirement of 'random" really means, and if you want your sampling to work, you must ensure that level randomization - especially the smaller the number of resident defects in the population. Just grabbing a handful of parts - assuming the process had randomized them - will not meet that requirement if the population is indeed stratified. It is clear that true random sampling will 'homogenize" the lot. For example, if lot sampling was from a rotating drum of bingo balls - and you pulled enough of them - you would be well represented of the probability of possible defects in the lot. That is the beauty of a perfect statistical example. If you can sample that randomly - yes, you will be in great shape. However, the reality sampling with such a clean randomization in an industrial setting (especially incoming receiving) is not always met - due to packaging considerations, etc. Anything less than perfect randomization can create sampling error causing erroneous conclusions.

So, the incoming distribution will not matter with sampling under those ideal conditions, and - conversely - you cannot determine the incoming distribution from sampling for similar reasons (those of you trying to calculate Cpks from incoming receiving sampling...not a good idea).
 
Last edited:
M

mjfqd

#14
Thanks for the ideas. I have asked the customer to specify the AQL and suggested the use of the C=0 plan.
 
Thread starter Similar threads Forum Replies Date
C Example Work Instruction/Procedure for AQL (Acceptable Quality Level) AQL - Acceptable Quality Level 4
S What are typical Acceptable Quality Levels (AQL) adopted in Food industry? Food Safety - ISO 22000, HACCP (21 CFR 120) 1
W Application of AQL (Acceptable Quality Level) Concept AQL - Acceptable Quality Level 15
M AQL (Acceptable Quality Level) for Mixed Part Defect IATF 16949 - Automotive Quality Systems Standard 1
S AQL (Acceptable Quality Level) Report Form example wanted AQL - Acceptable Quality Level 1
Q AQL - How do I determine Acceptable Quality Level (AQL) for In-Process Inspection? AQL - Acceptable Quality Level 2
A What is the purpose of using AQL (Acceptable Quality Level) for Sampling? AQL - Acceptable Quality Level 3
K So confused with AQL - How does AQL (Acceptable Quality Level) fit with my company AQL - Acceptable Quality Level 7
Y Advice on this real case on AQL (Acceptable Quality Level)? AQL - Acceptable Quality Level 3
L AQL (Acceptable Quality Level) Question Quality Manager and Management Related Issues 7
H Need help with AQL (Acceptable Quality Level) for a QC inspection AQL - Acceptable Quality Level 8
S AQL (Acceptable Quality Level) for Safety Critical Parts AQL - Acceptable Quality Level 13
N AQL (Acceptable Quality Level) Sampling Plan for Small Lots AQL - Acceptable Quality Level 8
H AQL (Acceptable Quality Level) for Medical Devices AQL - Acceptable Quality Level 14
Q AQL - How do I find Acceptable Quality Level? AQL - Acceptable Quality Level 75
D Problem with AQL (Acceptable Quality Level) AQL - Acceptable Quality Level 9
C ISO 9001:2000 In Textile Industries - AQL (Acceptable Quality Level) ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 2
Marc AQL - Acceptable (Assured) Quality Level AQL - Acceptable Quality Level 0
C Linking Acceptable Quality Levels (AQL) and Average Output Quality AQL - Acceptable Quality Level 7
apestate Custom hardware product - What acceptable quality level (AQL) is expected in the US? AQL - Acceptable Quality Level 4
J LTPD (Lot Tolerance Percent Defective) vs. AQL (Acceptable Quality Level) Definitions AQL - Acceptable Quality Level 5
N Setting AQL (Acceptable Quality Level) on C=Zero inspections AQL - Acceptable Quality Level 16
K AQL (Acceptable Quality Level) vs. IRR (Initial Reliability Requirement) AQL - Acceptable Quality Level 6
W Acceptable Quality Levels (AQL) - What is the usage of AQL 100 to 1000? AQL - Acceptable Quality Level 10
Marc AQL (Acceptable Quality Level) vs. PPM (Parts per Million) - Equivalences? AQL - Acceptable Quality Level 16
L AQL (Acceptable Quality Level) vs. LTPD (Lot Tolerance Percent Defective) AQL - Acceptable Quality Level 12
M Applying AQL when 0 defects are acceptable AQL - Acceptable Quality Level 9
W LTPD, AQL, Ppk and Cpk validation sampling plan table Inspection, Prints (Drawings), Testing, Sampling and Related Topics 0
R AQL, Consumer Risk and MA Statistical Analysis Tools, Techniques and SPC 2
A AQL - How to count samples with defects for each defect class AQL - Acceptable Quality Level 17
T Defining sampling plan for different AQL AQL - Acceptable Quality Level 3
S ARMY AQL - Requirements which don't have an AQL associated with them Manufacturing and Related Processes 2
M AQL table - I need to sample 1250pcs AQL - Acceptable Quality Level 3
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
I Sampling processes - Who must define the AQL level? AQL - Acceptable Quality Level 9
J AQL for Effectiveness Check - What AQL tiers to use on this type of sampling? Nonconformance and Corrective Action 9
S AQL Inspector's Rule - bring into CQE Exam? Professional Certifications and Degrees 5
N The question asks about sampling plans and AQL Reliability Analysis - Predictions, Testing and Standards 5
K Defining Acceptance Quality Level, I need clarity on AQL 1.5, 2.5, 4.0 AQL - Acceptable Quality Level 5
V Receiving Inspection AQL - Am I allowed to just pick a AQL? Inspection, Prints (Drawings), Testing, Sampling and Related Topics 10
L Established a c=0 "AQL" or "RQL" table AQL - Acceptable Quality Level 5
A AQL Sampling Plan SOP / WI - Medical Device Industry Inspection, Prints (Drawings), Testing, Sampling and Related Topics 14
L How to calculate the Probability of Deficiency of the AQL of an Inspection AQL - Acceptable Quality Level 16
P Setting Acceptance Quality Limits (AQL) for New Medical Devices ISO 13485:2016 - Medical Device Quality Management Systems 4
K AQL for Sterile Product - Laminate Tubes 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 5
M How to Mathemetically Derive the AQL Tables given Lot Sizes AQL - Acceptable Quality Level 5
L AQL ANSI z1.4 & z1.9 - What are the differences between these 2 standards? AQL - Acceptable Quality Level 5
P RQL/AQL from Attribute Data Six Sigma 1

Similar threads

Top Bottom