PQ Systems Knowledge Base

Category » Capability

There were 7 articles found in this category:

1. ST: Is Cpk the best capability index
   Revision Date: 2005-09-06 by Matt Savage Cpk has been a popular capability index for many years and perhaps because of its momentum it continues to remain popular. But is it the best index to use? Answering this question assumes that ther ...
2. ST: The capability index dilemma - Cpk, Ppk, or Cpm
   Revision Date: 2005-09-06 by Matt Savage Lori, one of our customers, phoned the other day to ask if Cpk is the best statistic to use in a process that slits metal to exacting widths. I too wondered what index would be best suited for her application. Perhaps Cpk, Ppk, Cpm, or some other ind ...
3. ST: How do we determine process capability if the process isn't normal
   Revision Date: 2005-09-06 Cp, Cpk, Pp, and Ppk do not necessarily rely on the normal distribution. If any of these indices increases, you know that the process capability has improved. What you do not know is how that improvement translates into good product. This requires knowledge of the ...
4. ST: Cpk or Ppk - Which should you use
   Revision Date: 2005-09-06 by Matt Savage Your customer has asked you to report the Cpk of the product you are sending. You know that to compute the Cpk, you need to have the product specifications, and that you need to have the mean and sigma. As you gather the information, someone asks, "w ...
5. ST: Should you calculate Cpk when your process is not in control
   Revision Date: 2005-09-06 by Matt Savage The AIAG Statistical Process Control reference manual (p. 13) states: "The process must first be brought into statistical control by detecting and acting upon special causes of variation. Then its performance is predictable, and its capability to me ...
6. ST: How can Cpk be good with data outside the specification
   Revision Date: 2005-09-06 by Matt Savage A customer who called our technical support line recently could not understand why his Cpk was above 1.0 when his data was not centered between the specifications and some of the data was outside the specification. How can you have a good Cpk when yo ...
7. ST: Calculating capability indices with one specification
   Revision Date: 2005-09-06 by Matt Savage The following formula for Cpk is easily found in most statistics books. Cpk = Zmin / 3 Zmin = smaller of Zupper or Zlower Zupper = [(USL – Mean) / Estimated sigma*] Zlower = [(Mean – LSL) / Estimated sigma*] Estimated sigma = average range / d2 A ...