If you have a team of inspectors in Asia, I bet they follow common sampling statistics (what people call the ‘AQL tables’, coming from the ISO 2859-1 standard or MIL-STD 105E). And your suppliers are used to them too...
Sampling Plan In Minitab
So you have been constrained by level I, level II, and so on. And sometimes the number of samples to check is very inconvenient for the inspector. It can force them to rush the job… or to take shortcuts… just to save appearances. Look, we followed the standard! It is really failed, you can’t discuss it!
The good news is, with specialized software such as Minitab, you can custom-make your own sampling plan while keeping the same key assumptions as ISO 2859-1 standard.
Let’s take an example. A batch includes 30,000 pcs, and the inspector can’t check more than 250 pcs in the allotted time.
- In level II, he would have to check 315 pcs. That’s too many.
- In level I, he would have to check 125 pcs. But you’d like him to go more in depth than that – ideally just under 250 pcs.
If you have access to Minitab 17, you can customize your own level!
Here is how to prepare this sampling plan in Minitab.
- Go to Stat > Quality Tools > Acceptance Plans by Attributes.
- Enter your settings and tweak the “Rejectable quality level” (RQL). Keep the other settings unchanged, so that the statistics are still compatible with ISO2859-1.
- Tweak the RQL and see the results. For example, you can increase the RQL to 6.3%, the sampling size is 242 pcs. The acceptance number for an AQL of 2.5 is 10 defective units.
For those you who are curious, what is the RQL?
It is what the ISO 2859 series of standards call the “limiting quality”. Here is its formal definition.
When a lot is considered in isolation, a quality level which for the purposes of sampling inspection is limited to a low probability of acceptance
In other words, the calculations underlying the ‘AQL tables’ establish the different between the inspection levels (S1, S2, S3, S4, I, II, III) by setting a different ‘limiting quality’ for each level. Once you can set the ‘limiting quality’ yourself, you can customize your own level.
Obviously Minitab doesn’t come cheap, but it can be useful to the quality engineer(s) in your organization. I hope this hack will be useful!
Do you use minitab? If so, what's your opinion? How did you create your sampling plan otherwise? Let us know your thoughts by leaving a comment below please.