The Cpk value is a measurement of how capable your process is of
producing materials that are within the within the specifications
promised the customer.
There are several ways to calculate the Cpk value. One common way is
as follows:
1. For each of the characteristics for which you have upper and
lower specification limits guaranteed to your customer, characterize
the variability in those characteristics in the product you produce
to ensure that it follows a normal distribution. (This is can be
done by making a histogram of the historic performance of your
product in each of the characteristics guaranteed to you customer,
and visually confirming that the shape of the histogram looks like a
bell curve. Alternatively, you can use statistical tools to measure
heteroskedasticity, skewness, symmetry, etc. and subject the data to
statistical tests to ensure that your process produces a "normal"
distribution in the variability in the output of the characteristics
of the products.)
2) Assuming that you have a process that is "in control" (i.e., a
process that produces a normal distribution in the characteristics
of interest to the customer), calculate the standard deviation in
the characteristics of interest to the customer (i.e., the
characteristics for which there is a specification value quoted to
the customer).
3) Calculate the average for each of the characteristics.
4) For each of the characteristics, find identify the specification
limit (upper limit or lower limit) that is closest to the average
value. Take the absolute value of the difference between the average
value and the closest specification value. Divide this difference by
the standard deviation calculated above, and then divide by 3.
The result is a factor known as the Cpk. Each of the continuous
characteristics (i.e., characteristics that can be measured on a
continuous scale) specified by the customer (or guaranteed to the
customer) should have a Cpk calculated for it. Because this Cpk
characterizes the distance from the average to the nearest spec
limit, it can be used as an indicator of how well the process is
able to fulfill the requirements of the customer.
Most customers want a Cpk of at least 1. What this means is that the
nearest spec limit is three standard deviations from the average. A
Cpk of 1, statistically, means that no more than 3 out of 1000
products will be out of spec.
The Cpk, however, has a far greater significance to your customers
than that: it means that you are performing statistical process
control, have characterized your processes, etc.
If you need more help in this area, getting your processes into
control will save you much money than it will cost you. Often the
best way to start with SPC (statistical process control) is to hire
a consultant who specializes in this area. My services are available
to you in this regard, or you can find other consultants readily on
the Internet (such as listed at ASQC site).
Good luck!
Warren
___________________
Dr. Warren Smith
[log in to unmask]
1-800-884-5387
JETS: Japanese-English Technology Services
* Japanese Technical Translation
* Quality Control Consulting
* Supplier Audits
-----Original Message-----
From: J. Gr $BaW (Balos <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Monday, March 22, 1999 2:20 PM
Subject: [TN] Cpk value
Hi Technetters,
A customer asks me for a "data measurement report and Cpk value".
What
does they mean with Cpk value?.
Your help will be extremely useful for me. Tks in advance.
Joaquin Gravalos.
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