What Sample Size for Destructive Test and is 'T' Test applicable?

M

Magus

#1
#1
I have a product that runs ~900 pcs per lot.
I have been running a destructive pressure test using a sample of 53 pcs per lot to try and show process capability. Sample size of 53 was derived from an 0.65 AQL sample c=0 as my initial sample size.

The mean of my sample test have been running ~1800 psi with a std dev of ~100 psi. I am trying to show that my total distribution is above a lower test limit of 1200 psi. (+/-6 sigma process). If 1 part of the sample tests below 1200 psi the lot is put on hold.

I am concerned about low side outliers. 450 psi is my customer’s high side failure threshold.

My amateur statistician is analyzing my work using a “T” Test. He states I can reduce my sample size to 7 pcs. Calculate the mean of the sample. If the sample mean of the 7 pcs is above 1500 psi the lot is OK.

I do not understand this application of the T Test. Is this valid and is the sample size of 7 correct?

Thanks,
Magus
 
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U

Umang Vidyarthi

#2
#2
Magus said:
I have a product that runs ~900 pcs per lot.
I have been running a destructive pressure test using a sample of 53 pcs per lot to try and show process capability. Sample size of 53 was derived from an 0.65 AQL sample c=0 as my initial sample size.

The mean of my sample test have been running ~1800 psi with a std dev of ~100 psi. I am trying to show that my total distribution is above a lower test limit of 1200 psi. (+/-6 sigma process). If 1 part of the sample tests below 1200 psi the lot is put on hold.

I am concerned about low side outliers. 450 psi is my customer’s high side failure threshold.

My amateur statistician is analyzing my work using a “T” Test. He states I can reduce my sample size to 7 pcs. Calculate the mean of the sample. If the sample mean of the 7 pcs is above 1500 psi the lot is OK.

I do not understand this application of the T Test. Is this valid and is the sample size of 7 correct?

Thanks,
Magus
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There is a similar discussion at 'This Thread' .

Umang :D
 
Bev D

Bev D

Heretical Statistician
Leader
Super Moderator
#3
#3
a t test is a test of means. it is used to compare either two means to each other or a single mean to a target value to determine if the mean(s) are statistically significantly different from each other.

It appears that your statistician is saying that as long as the mean of your sample is 3 SDs gretaer than your lower spec limit you have an acceptable lot.

it is not clear hwo he determined a sample size of 7

this approach may work but we would need to see your historical data first. can you post it in excel in time sequence?
 
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