# Estimation Sigma (Within) when subgroup n=1 (MiniTab)

#### Kastejon

##### Registered
Good afternoon.

Sorry for my English level.
I need help to understand the calculation of the standard deviation (within) And Cp estimated by minitab by using excel.

Attachment in excel example that I can not make (Sheet Cp).

Thank you.

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#### Kastejon

##### Registered
Sorry for I have seen the error on my part in data entry into Minitab. applying d2 = 1.12 I get cp.

The post can be closed

#### Steve Prevette

##### Deming Disciple
Staff member
Super Moderator
As far as I know, there is no such thing as a standard deviation for a "group" of one item. I can talk about the standard deviation of individuals within the population, but not a subgroup of one.

#### Kastejon

##### Registered
So I understand then it would be correct to calculate deviation using the formula (Sigma= R/d2) for individual samples?

#### Bev D

##### Heretical Statistician
Staff member
Super Moderator
If I understand the request correctly the within subgroup standard deviation is estimated using the moving range....
SD = average moving range / d2 where d2 is 1.182 (subgroup size' n=2)

#### Steve Prevette

##### Deming Disciple
Staff member
Super Moderator
If I understand the request correctly the within subgroup standard deviation is estimated using the moving range....
SD = average moving range / d2 where d2 is 1.182 (subgroup size' n=2)
True, maybe that is what they are asking . . .

#### Kastejon

##### Registered
Yes, my confusion or doubt comes from the Minitab software documentation.

Minitab:

"Estimating σWithin when subgroup size = 1
Average moving range ? Default
Withinσ?= StDev(Within) = ()wdRM2
MRi = the ith moving range = max[xi, ?, xi-w+1] ? min[xi, ?, xi-w+1], for i = w, ?, n RM= ()()1...+−++wnMRMRnw
w = the number of observations used in the moving range. w = 2 by default."

However I have managed to solve my problem. Thank you for your comments and assistance and sorry for my English.

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