Normal Distribution Queries

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• #51025

Tinoco
Participant

Hey just have some queries on normal distribution if anyone can help

Normal distribution is used to illustrate????? Cause variation.
And Normal distribution describes what????% of the variation in the process
And is variation that exceeds normal distribution is due to????? Causes.

Thanks All

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#176226

Robert Butler
Participant

This is not meant to be condescending nor am I trying to start a flame war but your post suggests you do not have a clear understanding of the concepts of variation and distribution. The answers I’ve offered below are reasonable with respect to your specific questions but they are not the answers to the questions I think you meant to ask.
A histogram of data from a normal distribution will provide an illustration of the variation of a process whose output is normally distributed – ordinary or special cause have nothing to do with it.
If a process is normally distributed and if you make a histogram of the data then that histogram will illustrate the process and provide a visual assessment of the variation.  For the sample in question the illustration will provide a picture of 100% of the variation of the sample.
Variation doesn’t exceed a normal distribution or any other kind of distribution.  Variation is a measure of the spread of a distribution.

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#176324

Whitehurst
Participant

In regards to query three special causes is a source of variation that causes a fundamental change in a process. Special causes distort a normal distribution.

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#176329

Robert Butler
Participant

The statement “Special causes distort a normal distribution. ” is in error.
Normality does not imply an absence of special causes and non-normality does not imply the presence of special causes.
For the case of normal data with special causes present see the post below:
https://www.isixsigma.com/forum/showmessage.asp?messageID=123488
As for the second – a process with a physical bound will produce non-normal data even when special causes are absent. As your in control process approaches the physical bound the non-normality of the data will increase and that non-normality will have nothing to do with special cause.

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