Customer Inspection Frequency and Rejection Level Criteria - Sample Lot Size

B

bladeblaster

#1
Hi All,

This is my first post, but I have had a good look around the forum for my answer, but without a huge amount of luck. This my be due to a lack of understanding exactly what I am looking for though. :eek:

Here is my quandry, I have requested from my customer an inspection frequency and rejection level criteria for a moulded part that we produce. My customer has responed stating that an AQL of 0.65 and that the acceptance level is zero defect in sample lot.

Now is this enough information for me to determine the sample lot size? Typically out manufacturing lot size will be 200,000 parts, I believe from some of the information I have found on here that in this case our sample size should be 119 parts, is this correct?

Many thanks.
 
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Tim Folkerts

Super Moderator
#3
You should be able to work backwards through the Z1.4 standard. For AQL = 0.65 and Ac = 0, Table II-A for normal inspection say you should draw a sample of 20 (Line F).

You would then look back to Table I to determine the inspection level. However, "F" does not exist for a lot size of 200,000. So you would either be using "S-2" or "S-3" inspection level -- you can take your pick since either E or G will lead back to a sample size of 20. (Personally I would be tempted to call it S-3 since that is more rigorous in general and hence would sound better. :))
 

Mike S.

Happy to be Alive
Trusted Information Resource
#4
Blade,

Looks like you are using a true C=0 sampling plan (i.e. Squeglia-type). The sample size you propose (119) would fit the criteria your customer specified.
 

Tim Folkerts

Super Moderator
#5
[ WARNING: This answer has some incorrect calculations -- see the correction a couple posts later! ]

Mike's answer (119) would be the size to be pretty sure that 0.65% defect rate (or higher) would be rejected. My number (20) is the size to be pretty sure 0.65% (or lower) would be accepted.

In other words, if the actual quality was right at 0.65% defective, then n=119 will probably reject the lot; n=20 will probably accept the lot (with around 5% chance in each case of making the wrong decision).

You have to decide what it is you want to accomplish with your sampling plan.
 
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qusys

Trusted Information Resource
#6
Mike's answer (119) would be the size to be pretty sure that 0.65% defect rate (or higher) would be rejected. My number (20) is the size to be pretty sure 0.65% (or lower) would be accepted.

In other words, if the actual quality was right at 0.65% defective, then n=119 will probably reject the lot; n=20 will probably accept the lot (with around 5% chance in each case of making the wrong decision).

You have to decide what it is you want to accomplish with your sampling plan.
Hi,
could you please provide some links or files where it is possible to check these values with a brief explanation to get the above result?:thanx:
 

Tim Folkerts

Super Moderator
#7
Wow -- it looks like my initial calculations were WAY off! (Actually, I knew that n = 20 might not be quite right since Z1.4 is not based closely on statistical calculations. But somehow I mis-calculated the other number badly.)

The probability, p, of finding no defects in a sample of size "n" with a defect rate "d" would be
p = (1-d)^n

after playing with logarithms a little, this can be rearranged to give

n = log(p) / log(1-d)

So for d = 0.0065 (ie the defect rate equals the AQL of 0.65), and p = 0.95, we get n = 7 So if you want to accept lots with AQL = 0.65 at least 95% of the time (ie reject no more than 5% of the time), you should only sample 7 items!

So for d = 0.0065, and p = 0.05, we get n = 460. So if you want to reject lots with AQL = 0.65 at least 95% of the time (ie accept no more than 5% of the time), you should sample 460 items!
 
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