How I calculate Standard Deviation
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 This topic has 4 replies, 5 voices, and was last updated 14 years, 6 months ago by Bill Fowlkes.

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May 15, 2008 at 9:48 pm #50090
How do I calculate the standard deviation when the USL and the LSL are known ? I would like To see an example if someone has one and is willing to share. This is a new experience for me.
Thanks for any help,
Brew
0May 16, 2008 at 4:15 am #172014
B KumarParticipant@BKumar Include @BKumar in your post and this person will
be notified via email.Hello,
In this case, do you have sigma level. If you know the Sigma level, you can calculate sigma value as below.
Sigma Value = (USL – LSL)/ Sigma Level.
If your data is continuous data
Sigma Value = (UCL – Mean)/3;
or Sigma Value = (Mean – LCL)/3
UCL and LCL are Upper and lower Control limits.
Suppose, if your data is Attribute data, then
Sigma Value = NORMSINV [ (1 – DPO) +1.5 ]
DPO = Number of defects / Total opportunities for eror.
So, generally USL – LSL = Sigma Level * Sigma Value.
By
Ravi B0May 16, 2008 at 4:55 am #172017
sandrine berthetMember@sandrineberthet Include @sandrineberthet in your post and this person will
be notified via email.Not enough
You have to know the Cp to use the equation and to figure the sd0May 16, 2008 at 7:06 am #172022
aiman faridParticipant@aimanfarid Include @aimanfarid in your post and this person will
be notified via email.You can’t calculate the standard deviation when only the USL and the LSL are known. As the USL and the LSL are specification spread.
To calculate standard deviation you need real data or the UCL and the LCL as actual process spread.
Or, when you know the relationship between both (process and specification spread)
The USL and the LSL + capability index Cp (no shift) or Cpk (shifted)
The USL and the LSL + Sigma level (number of standard deviations lying between the mean and the nearest specification limit)
Aiman Farid0May 22, 2008 at 2:16 pm #172154
Bill FowlkesParticipant@BillFowlkes Include @BillFowlkes in your post and this person will
be notified via email.Ravi, this is so wrong. If it was true that sigma = (mean – LSL)/3, or better sigma = (USL – LSL)/6, then all Cp would equal 1 and everyone would have the same defect rates. Of course that is not true.
If fact, the USL and LSL should be related to customer requirements, and the sigma to manufacturing varaibility. So, the correct answer is that you CAN NOT calculate the standard deviation from the spec limits. Those are two very different things.0 
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