What is a Good Sampling Plan for Small Batches (10-250)? (2024)

1

1winnyAK

  • Jun 22, 2010
  • #1

Last time I was here, I wanted to create a sample plan using the AQL Sample plan and now I don't think I am doing it right because my batches are so small and I want 95% of the time the parts are good or 99%. I found a post on here that got me confeused.

Can someone please help me explain to me I'm I going the wrong direction on my sample plan approach?

"CAPABILITY MEASUREMENTS like Cpk and Ppk are typical measures of how well you are actually doing. You calculate these by looking at your measured results, finding the mean and standard deviation, and then doing some calculations comparing the mean and standard deviation to the specs. (And as Bob would rightfully point out, for precision machining you might want to do the calculations slightly differently than you would for other processes.)

A typical rule of thumb is to measure at least 30 pieces. More is better, but 30 gives pretty good results. Because capability calculations are based on variable data (ie numbers that can take on many results over the range of interest), a relatively small number of measurements (like 30) will give clear and effective results. These sorts of capability measurements are well known and well accepted in many industries.

This should be done in conjunction with control charts to see that the processes is behaving consistently.

AQL SAMPLING PLANS, on the other hand, are based on a value you choose. Basically you are say "if the parts meet the chosen AQL (or are better), then the lot will usually be accepted, and if the lots are noticeably worse than the chosen AQL, then the lot will usually be rejected." In your case, an AQL sampling plan at the end (or when they receive the parts) could be effective as insurance on top of your other measurements. For example, if a shipment got mislabeled, a sample of a few parts would quickly spot the error.

Because AQL plans use attribute data (eg pass/fail), they require large samples if you want a high degree of certainty. You might check this thread for info Confidence and Reliability - Reference to a 90/95 confidence and reliability level. For exampl, if you wanted to be 95% certain that at least 99% of the parts are good, you would have to sample 299 parts! Since you don't make this many parts, then this is not effective. But 30 parts for an estimate of Cpk could give you a similar certainty.

Let me recap my opinions on the matter:

  • SPC (control charts) shows that you are being consistent
  • Capability (Cpk) shows how well you actually have done relative to specs.
  • Sampling (AQL) provides a rough check of how well you have done "

This is where I found the post; AQL - How do I find Acceptable Quality Level?

What is a Good Sampling Plan for Small Batches (10-250)? (1)

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bobdoering

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Trusted Information Resource

  • Jul 2, 2010
  • #3

If you are looking for a sample size for your own process, I do not recommend using acceptance sampling plans designed for incoming product from someone else's process. They use the wrong logic. I believe you would be better off using discovery sampling plans, as they are designed to increase your sample size as your defect rate becomes less. This is important because you want to be able to detect a defect if you have an idea that there is a probability it can exist. To calculate the sample size, try the Discovery Sampling Plan button on the linked spreadsheet. It utilizes defect rate (capability), confidence level, and lot size as factors.

K

kabus - 2011

  • Jul 13, 2010
  • #4

I hope I am doing this right. This is my first time here. I do have a similar question. I am trying to develop a sampling plan for a packaging line (pharma industry) that produces bottles and blisters. I have the types of defects that shall be inspected for and the corresponding AQL. However, I need to come up with the sample size and the frequency of inspection. I used ANSI to develop the plan, but for critical defects (AQL = 0.01%) it requires inspecting 1250 What is a Good Sampling Plan for Small Batches (10-250)? (4). This is a large sample size for lots of 7,000 - 8,000 units that are produced in 8 hrs. I have read a lot of threads about sampling plans, and I would like to know the difference between using ANSI or the OC curve (alpha / beta risk). Is there a difference statistically? Which one is preferred by FDA? Also, do the alpha and beta risk is soemthing agreed by the company or is there a norm? Any info or recommendations will be very appreciated!!!!

bobdoering

Stop X-bar/R Madness!!

Trusted Information Resource

  • Jul 13, 2010
  • #5

kabus said:

I hope I am doing this right. This is my first time here. I do have a similar question. I am trying to develop a sampling plan for a packaging line (pharma industry) that produces bottles and blisters. I have the types of defects that shall be inspected for and the corresponding AQL.

Do you have a known frequency of defects (probability)?

K

kabus - 2011

  • Jul 13, 2010
  • #6

No, what I have is the defect classification (minor, major, critical) and the AQL that applies to each. What I meant by frequency was that I need to determine how often (i.e. every 30 min), the samples shall be collected for inspection.

S

Sturmkind

  • Jul 13, 2010
  • #7

Dodge & Romig offers both single and double sampling plans that generally have smaller sample sizes than the ANSI/ASQ tables. Wiley Classics library -Sampling Inspection Tables.

However, in the words of the late Robert Dovich on 12/15/94 "...quality is...a business decision, NOT a dedication to esoteric values.". If a product defect is truly critical, then he posited that you either do no inspection because the line is sufficiently controlled and error-proofed or 100% automated inspection & sort (vision, scanning, etc.).

K

kabus - 2011

  • Jul 14, 2010
  • #8

Anyone has the Dodge & Romig tables to share? for single and double sampling plans. Thanks in advance!!!

Bev D

Heretical Statistician

Leader

Super Moderator

  • Jul 14, 2010
  • #9

Not sure you need the Dodge and Romig tables yet. (A google search will quickly return various sources of these tables for purchase)

What is the reason for your sample inspection? are you monitoring for process control or performing 'release' testing on a continuous stream of production (as opposed to randomly sampling the completed lot)?

where did the AQL value(s) come from?

K

kabus - 2011

  • Jul 14, 2010
  • #10

Random sampling to a completed lot, mostly for packaging defects.
The AQL was pre-established based on the defect classification (minor, major, critical), which in turn was assigned based on the definition of each classification.

Do you have any recommendations?

Like I mentioned before, I am trying to develop a sampling plan that is suited to the process. We are talking about a packaging line (lot by lot) that produces from 7,000 to 16,000 units per lot in an average run time of 8 hours. Applying a normal inspection level II ANSI sampling plan requires too many samples. I am trying to look for an alternative that is still statistically reliable and scientifically sound.

Any help will be apprciated!!

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