# New to Six Sigma – need help on statistics

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• #37579

G R
Participant

I have made numerous aborted attempts to learn Six-Sigma concepts and apply them, however, there are so many statistical terms that tend to unsettle me. I have taken a basic course in statistics but that doesn’t seem to help much – I still can’t seem to grasp the concepts behind Six-Sigma. E.g. 1) what does a sigma REALLY mean ? 2) what is a z score? can someone explain in easy to grasp examples?
Is there some website (university study site etc.) that offers a introduction to statistics in understandable language
Is there some website (university study site etc.) that offers a introduction to SIX-SIGMA using easy to grasp real world examples?
All help would be truly appreciated.

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#110898

Whitehurst
Participant

You know what the “average” of some data is.  To get the average you add all the data together and divide by how many numbers you had.  How much variation is in your data?  For teaching purposes, I like to call “sigma” the average amount of variation in your numbers. Take the average of the data and subtract it from each one of your data entries.  Square each of the differences you get.   Sum up all your squared differences to get a total.  Divide this total by the number of data entries you started with to get an average.  This is the average amount of variation…squared.  Take the square root to get “sigma”.  Sigma is another name for STANDARD DEVIATION.  It is the average amount of variation in your data.  The larger sigma is, the greater is the amount of variation in your numbers.  This explanation is for 30 or more individual pieces of data.  If you have less data, divide the sum total of squared deviations by the amount of data you had minus one.  (20 samples? Divide by 19)  There is an altogether different calculation for subgroups of data which I will not go into.  The definition, the logic, the probabilities, and the effect of the variation has on the width of the histogram, the upper and lower limits (99.73%) are all the same regardless if your data is individual data entries, more than 30, less than 30, or sampled by subgroups.  The logic remains the same, the calculations change for subgroup sampling.  This is a good beginners deffinition.  Standard deviation, or sigma, is a measure of the average amount of variation in the data.  Now you can compare not only averages of different groups of data, but you can also compare the amount of variation in each group.

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#110931

G R
Participant

Hello Joe,
Thank you SO MUCH for taking time to write the explanation. I now understand it better. Do you have any site links you can share with me that offer easy explanations of Six-Sigma and statistical terms, explanations such as the one you shared with me?
Thank you & have a nice day!

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#110958

Whitehurst
Participant

GR, perhaps I can be of assistance.  I am willing to help for free.  Contact me at [email protected].

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#110963

Ron Manubay
Member

Hi GR,I think that part of the confusion in learning statistics and the Six Sigma concept introduced by Motorola, is the fact that the meaning of SIGMA differs between the two.Joe has given you an excellent description of what SIGMA is as a statistical term. It is – standard deviation or simply, the average deviation of each data in your sample from the mean. Six Sigma concept is different. In the context of a manufacturing process where you have a process distribution and spec. limits, SIGMA in this case, is the number of Std.Deviations your process mean is away from your nearest spec limit (either LSL or USL). So, the more “sigma” you have, means the farther your process mean is from the nearest spec limit. This translates to much fewer rejects even if your process distribution shifts. Here’s where the confusion starts. If you are talking about sigma in statistical context, the lower it is, the better, since it means better precision or repeatability. On the other hand, if you talk about sigma in the context of the Six Sigma concept, the higher the sigma, the better, since it talks about the number of sigma your mean is away from your nearest spec limit. Did I confuse you more?Regards.

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#110966

Johnny Guilherme
Participant

Ron
Can you go through that last sentence “On the other hand…….nearest spec limit”. Now I am a little confused.
Johnny

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#110967

Ron Manubay
Member

Ronnie

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#110968

Johnny Guilherme
Participant

Ron
Thanks-it was long but its makes sense. Thanks for clearing this up for me.
Johnny

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#111085

Ron Manubay
Member

Hi Johnny, No problem. I’m glad to be of help.Ronnie

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#111089

Goulais River
Participant

I have made numerous aborted attempts to learn Six-Sigma concepts and apply them, however, there are so many statistical terms that tend to unsettle me. I have taken a basic course in statistics but that doesn’t seem to help much – I still can’t seem to grasp the concepts behind Six-Sigma. E.g. 1) what does a sigma REALLY mean ? 2) what is a z score? can someone explain in easy to grasp examples?
Is there some website (university study site etc.) that offers a introduction to statistics in understandable language
Is there some website (university study site etc.) that offers a introduction to SIX-SIGMA using easy to grasp real world examples?

GR,
I am curious as to what your bussiness is and why you think you need 6sigma. Are you currently doing any other continuous improvement initiative?
If you have been reading up and taken courses, and still cant see what it’s all about, I bet you don’t need 6sigma, and all the statistacal BS that comes with it.
Just a thought.
GR

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#111100

SemiMike
Member

If you are TOLD that you need this, then begin the long slow journey of undertstanding by IGNORING mathematical statistics and just jump in and GRAPH THE DATA.   After many attempts at graphing data over time, by machine, by part, by site within part, etc etc you may begin to see why people use DESCRIPTIVE statistics to talk to each other.
Then try IMPROVING a process by making small adjustments and plotting the results.  After a while you will see why people invented Design of Experiments, Anova, Mulitple Linear Regression, and other tools for INFERENTIAL statistics.
But if you must learn fast, start with a class in an Industrial Engineering department, not a Math department…in my opinion.  And Six Sigma is more about PROJECT MANAGEMENT and TEAM LEADERSHIP and INDUSTRIAL KNOWLEDGE than it is about statistics, IMO.
Web sites that help:
http://www.ruf.rice.edu/%7Elane/rvls.html  basic with animations
http://www.itl.nist.gov/div898/handbook/  higher level

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