And just for what it's worth, unless you understand what you're measuring, don't use Cp's or Cpk's (he said to the horrors of the masses!!). This is because of the dependency of Cpk on the specifications or limits. You can artifically drive the Cpk number higher merely by widening the limits. It also has a tendency, bu itself, to suggest a process is in control, when in fact it is out of control. Take for example a process which has a Cpk of 2; with a purely bimodal distribution at the plus and minus 1.0 sigma limits, one can get such a Cpk; however, I would submit to you that the process which produced one mode is different that the process which produced the other mode. You may get an inkling of bimodality by including a first order statistic along with Cpk, such as range or standard deviation. Probably the best method is to understand what the Pearson number is (or Pearson set, as it's genrally called). This is nothing more than a plot of skew v. kertosis and will tell you histogram shape and a value which is akin to Cpk called the limit (either LL or UL). Enough of stats101... > Bill Davis, Ph.D. > Diamond Multimedia Systems > Senior Scientist > Tel. 408.325.7868 > Cell. 408.888.5650 > e-mail: [log in to unmask] > > -----Original Message----- From: [log in to unmask] [mailto:[log in to unmask]] Sent: Wednesday, February 24, 1999 2:14 PM To: [log in to unmask] Subject: [TN] Cpk Question Isn't the world of statistics wonderful. At some very large corporations (often automotive), Cpk is equated almost to the bottom line. A Cpk simply provides some evidence of how well a process is performing relative to whether it is in control (first) and whether it is capable of producing product meeting specified requirements (second). This requires a processes' control limits to be inside the product's specified acceptance limits. If you were to turn a Cpk chart on its side and match its control limits to to those of an X bar R chart, or process control chart, you would get a clear picture whether this was true. The higher the Cpk's numeric value, the better is a process capable of performing. Also, remember that when a process is said to be in control, this only means it is consistent. That is, it may be effecting product either consistently good - or bad. When within the Cpk's specified limits, it is said to be in control and effecting product meeting specified requirements. That's a very short version of it. Earl Moon ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################ ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################