From: Stan Hilliard
Date: 08 Jun 2000
Time: 00:27:11
Greetings Paul,
C is a symbol for acceptance number of an acceptance sampling plan for attributes. C is called Ac in some systems, like Z1.4, but they mean the same thing.
The acceptance number cannot be used to estimate the fraction defective. It is not data. It is a policy, only used to decide whether to accept or reject the lot. The two items, sample size (n) and C, make a sampling plan -- which is a decision rule.
If your customer has an estimation procedure related to C, you might find out exactly what operations and calculations they do. It sounds unusual to me.
To estimate fraction defective, you divide the actual number of defectives by the total number of units.
If you want an interval estimate, you could calculate the 95% confidence limits about the point estimate. That would tell how precise or imprecise the estimate is.
If your customer finds and returns all of the actual defectives, then what you now calculate is the fraction defective. Multiply that by 1,000,000 and you have parts per million defective. (Oops, I wasn't going to get mathematical)
If some defectives get by your customer, then what you are measuring is the fraction of returned defectives, which would be smaller than the actual fraction defective received.
Sincerely, Stan Hilliard