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How Many Samples Do You Need to Be Confident Your Product Is Good?

A simple formula gives you the sample size required to make a 95-percent confidence statement

Jim Colton
Tue, 10/06/2015 - 12:54
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How many samples do you need to be 95-percent confident that at least 95 percent—or even 99 percent—of your product is good? The answer depends on the type of response variable you are using—categorical or continuous.

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The type of response will dictate whether you’ll use:
1. Attribute sampling: Determine the sample size for a categorical response that classifies each unit as good or bad (or perhaps, in spec or out of spec).
2. Variables sampling: Determine the sample size for a continuous measurement that follows a normal distribution.

The attribute sampling approach is valid regardless of the underlying distribution of the data. The variables sampling approach has a strict normality assumption but requires fewer samples. I’ll focus here on the attribute approach.

Attribute sampling

A simple formula gives you the sample size required to make a 95-percent confidence statement about the probability an item will be in spec when your sample of size n has zero defects.

 …

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