Long ago I read from an unknown source a break down of dollar amounts that a
small, medium, and large sized company would invest towards ISO 9000
certification. I have been searching the internet trying to find this
information. Would you have the answer to this question or know how I could
find out?
firebrew 9/24/2001
Actually, you can indeed estimate sigma and calculate conttrol limits by lumping all the data points into one theoretical distribution. If you do however, you are no longer "doing" SPC.
I'm not sure what you are doing, but it ain't SPC.
The AIAG describes a methodology of doing this to estimate what they call a "Process Performance" index.
It allows you to describe the results of a process evalutation without making any assumptions about process stability or statistical control. The results of such an analysis are accurate for that one frame, but because statistical control is not verified, it is not valid as a predictor of future performance.
In nearly all cases, when you estimate sigma from the individual data points, and then calcualte 3 sigma limits, you will enclose all the data points within the limits and you can easily assume stability when it may not be there.
It's exactly the same as doing a "Statistics 101" frequency distribution analysis like you learned in your first semester class in college.It characterizes a pile of data using statistical words, but cannot predict future performance.
J Bruman
guest 9/12/2001
The objective of a process behavior chart is to separate the routine variation from the exceptional variation. The only reliable way to filter out the routine variation is to use the within-subgroup variation to compute the limits. Using global, descriptive measures of dispersion will mix both types of variation together and make it impossible to separate the two. Thus, all the correct ways of computing limits for process behavior charts use local, within-subgroup measures of dispersion rather than the global, descriptive measures.