Sample Size for 2900 ppm Visual Defects Inspection Requirement

G

grs8282

#1
Hello!

I have a customer that requires a defect rate under 2900 ppm for visual defects (so parts are either good or bad). Our lots are typically 10,000 pieces. What formula/test should I use to figure out my sample size to be 99% confident we will meet the requirement?

I tried using the "Acceptance Sampling By Attributes" in MiniTab, but I am not sure what to put in the AQL and RQL fields...

Let me know if you need more info!

Thank you,

~Gabriela
 
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Bev D

Heretical Statistician
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#4
Without regard to AQL (which your custoemr seems uninterested in and rightly so) an RQL of 2900 ppm with 99% confidence woudl give you a sample size of 1586. the lot would be accepted if there were no defects in the sample and rejected for 100% screen if there were 1 or more defects in the sample.

I used the exact Binomial equation (sample sizes for acceptance sampling are typically not affected by lot size unless the lot size is fairly small; <50 or so)

The formula for the exact Binomial (reject on 1 or more defects) is:

n= LN(1-P(detection))/LN(1-p)

P(detection) = confidence level which in your case is .99
p = defect rate which in your case is 2900 ppm or .0029
LN is the natural log
 
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