A Capability Study for an Entire Product


Bothe, Davis R.   (1992, ASQC)   International Quality Institute, Northville, MI 48167

Annual Quality Congress, Nashville TN    Vol. 46    No. 0
QICID: 9833    May 1992    pp. 172-178
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Article Abstract

The text of this paper is the same as QICID 9627. Do not order both.

Many methods exist for finding process capability information for a single product characteristic. Often, though, customers are interested in more than one critical characteristic. You can combine the capability goals of several characteristics to determine if an entire product will satisfy the customer.

Product A, for example, has three critical characteristics. All three must be capable if the product is to satisfy the customer, thus only one bad characteristic can render the product unacceptable. To test its total capability, first stabilize the first characteristic's process, then calculate the Lower and Upper Specification Limits (LSL and USL, respectively) for that characteristic, as appropriate. Use these figures and the table of standard normal deviates to calculate the probability that this characteristic is out of specification. Repeat for the other two critical characteristics. Calculate the product of all three capabilities to find the product capability.

Using the same set of formulas used to calculate individual characteristic capability, you can combine characteristics into products, and operations into assemblies. This allows you to evaluate and compare different products on their ability to meet a range of customer requirement, rather than a single characteristic.


Control charts,Process capability index (Cpk),Standard deviation,Automobile industry

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