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Determining sample sizes for PQ

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Miner

Forum Moderator
Staff member
Admin
#2
Can you give us more context and detail? What are you trying to do? This appears to be unrelated to the original thread topic, so we don't have much to go on.
 

Johnnymo62

Haste Makes Waste
#3
I ended up doing capability on each cavity individually because the cavities varied too much, compared to each other, to be capable on many of our injection molds.
 

SylvieS

Starting to get Involved
#4
Can you give us more context and detail? What are you trying to do? This appears to be unrelated to the original thread topic, so we don't have much to go on.
Sorry - I agree that my question was oddly phrased.
If I have a multi-cavity tool - let's say 2 cavities, to do the PQ I will evaluate using the Binomial Law the quantity n of sample required for a specific confidence and reliability level I wish to achieve.
My doubt is about the unit for n. Should the unit be 1 shot/run of my tool (1 unit / n=1 / 2 cavities => 2 products) or n=1 is 1 product?

I hope it clarifies.
 

Miner

Forum Moderator
Staff member
Admin
#5
This is a common scenario, but interestingly enough, it is never addressed in texts/manuals on statistics. This may have a significant impact on the concept of rational subgroups and the warning against mixing process streams, but no guidance is given on the appropriate way to handle it.

So here are my thoughts on the subject. To keep it simple, we will continue your 2-cavity mold scenario. The two cavities will not be identical, and therefore may represent two process streams. If they are indeed different, they should not be included in the same subgroup and should not be combined into the same capability study.

My recommendation would be to collect samples from each cavity and perform a 2-sample t-test on the cavity means. If the t-test is statistically significant, treat the cavities as different process streams. If the test is NOT statistically significant, treat them as a single process stream. You may safely include them in the same subgroups.

The next thing that I would consider for your specific question is, what is the major source of variation? Is it the variation between the two cavities? Or is it the variation from heat to heat? The samples should focus on the major source of variation that has the potential to vary over time. The difference between cavities will likely not vary until a long time passes, while heat to heat variation may occur over a much shorter time frame.
 
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