Hello Fellow IE's: Had a question
Hello Fellow IE's: Had a question
(OP)
Hello Fellow IE's:
Had a question regarding data that appears to be somewhat bell-shaped curved, but I do not recall if there is a definite calculation that will tell me whether or not the population follows a normal distribution. I am looking at starting SPC at my site on our sortation system (Rapistan)and I want to begin by looking at CP/Cpk, Pp/Ppk. I know the formula changes for Cp's and Pp's if it's non-normal, but before I dig around for non-normality formulas I want to ensure that I do not have a normal distribution. I am evaluating numbers of sortation errors vs case volume. Thanks for your help.
Had a question regarding data that appears to be somewhat bell-shaped curved, but I do not recall if there is a definite calculation that will tell me whether or not the population follows a normal distribution. I am looking at starting SPC at my site on our sortation system (Rapistan)and I want to begin by looking at CP/Cpk, Pp/Ppk. I know the formula changes for Cp's and Pp's if it's non-normal, but before I dig around for non-normality formulas I want to ensure that I do not have a normal distribution. I am evaluating numbers of sortation errors vs case volume. Thanks for your help.





RE: Hello Fellow IE's: Had a question
Normal density curves satisfy the following property which is often referred to as the Empirical Rule. Almost all values lie within 3 standard deviations of the mean for a normal distribution.
68% of the observations fall within 1 standard deviation of the mean.
95% of the observations fall within 2 standard deviations of the mean.
99.7% of the observations fall within 3 standard deviations of the mean.
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Also, in a normal distribution, the mean, median, and mode are equal so that is a quick test of normality.
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Look at step two from:
http://www.netgm.com/tools/useful_stuff/process_capability.html
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Remember, large sample theory says basically that the larger the sample size, the closer one approaches a normal distribution. See: http://www.stat.sfu.ca/~lockhart/richard/450/99_3/lectures/13/web.html
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More links on normal distributions:
http://ce597n.www.ecn.purdue.edu/CE597N/1997F/students/michael.a.kropinski.1/project/tutorial
http://www.stat.wvu.edu/SRS/Modules/Normal/normal.html
http://davidmlane.com/hyperstat/normal_distribution.html
http://mathworld.wolfram.com/NormalDistributionFunction.html
http://www.itl.nist.gov/div898/handbook/eda/section3/eda3661
RE: Hello Fellow IE's: Had a question
RE: Hello Fellow IE's: Had a question
On the first link you gave me it shows five steps to calculate process capability. I am confused about one thing though. My data showed that it was non-normal so I went ahead and transformed it by grouping the data n=109 samples into 3 groups, took the average, and calculated the Z value of the transformed data and of course the transformed Z plot was a straight line since averages always have a normal dist. In order to calculate my process capability now do I have to calculate the USL and LSL, X-bar, and std. dev. using just the 3 averaged data points or the 3 transformed Z values in order to come up with Cp and Cpk? or did I transform it just to graph which it wouldn't make sense cause I still need my Cp calculation. Please advise the website is not clear. Thanks.
RE: Hello Fellow IE's: Had a question
Do you have any SPC books? I have the Pyzdek book somewhere, but I'm not sure if it's with me here or in Texas. I'm going to be offline tomorrow - have to drive up to Chi-town to pick up DS' "new" car so it may be a few days.
In the meantime if you have a time crunch, don't wait on me, hit a library and see what you can find on SPC.
RE: Hello Fellow IE's: Had a question
AxelCS