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bkarthikeyan
When using SPC how to identify whether my data follows normal or non-nroaml ( without using minitab or softwares). Can we use Histogram to find these?? If so any min. no samples to be taken??
Dear Mr BradM,
I was explaining SPC concepts to one of the fresher to my org. During discussion this question came. I know that through Minitab we can check whether data is normal or not . If we dont have Minitab then how to find out ?? In our factory our Mfg process incudes Stamping, Painting, Assembly so I want to know how much data needed because each process output rate varies.
Dear Mr BradM,
I was explaining SPC concepts to one of the fresher to my org. During discussion this question came. I know that through Minitab we can check whether data is normal or not . If we dont have Minitab then how to find out ?? In our factory our Mfg process incudes Stamping, Painting, Assembly so I want to know how much data needed because each process output rate varies.
If you collect 1500 data points, normality will virtually be a given.
There are a number of statistical tests for normality. Some of the easiest are based upon if the skewness and kurtosis are the proper value for the normal distribution.
The most common test (and can be implemented with a little excel programming) is a chi-square test on the distribution of the data into histogram bins as compared to what the normal distribution would predict for the bins.
One comment - this statement is only true if you are dealing with the average of the 1500 values. If you are dealing with the "tails" of the distribution (such as trying to predict failure) normality is definitely not a "given". I think this is what fooled the developers of six sigma into thinking there was a 1.5 sigma shift.
There is a good book out there by Nassim Taleb called The Black Swan. It's an interesting book, written in plain language. Tom Peters has been highly supportive of his works. The Black Swan does show how we are often fooled by assuming normality, and by our reactions to rare events.
Hello Steve,
Great suggestion, but the poster, bkarthikeyan, indicated that it needed to be determined without Minitab or other software. I assume that they may not have access to these resources. So it's back to basics....
You know? Now I'm thinking of it, that's what the (SPC) Masters did...
Stijloor.