# usage of X-bar R Xbar s chart

Six Sigma – iSixSigma Forums Old Forums General usage of X-bar R Xbar s chart

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• #50353

fresher
Participant

Hi Team ,
will appreciate if any one help me in understanding the difference of -X bar R & Xbar s chart , and their usage, i mean when can u use which .
regards
fresher

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#173076

Bower Chiel
Participant

Hi FresherBoth chart pairs do the same job. In his book Introduction to SQC Doug Montgomery advises using Xbar and s charts rather than Xbar and R charts 1. When sample size is greater than 10 or so and 2. When sample size is variable.
Best WishesBower Chiel

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#173081

fresher
Participant

hi Bower,
tnx for u r input , so should i say for a varaible sample of more than 10 one should use Xbar-s & for lesser sample we should use Xbar-r ,
hope i have understood u r guidance propely ,
regarsds

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#173082

Ax Campos
Participant

Hello
Another criteria is to use X-R charts whenever you have to make the calculations by hand directly at the work station. The reason for this is that it is a lot easier to calculate Range than Standard Deviation.

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#173083

fresher
Participant

thnks & whar abt for xbar -s

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#173091

Ron
Member

Xbar & R charts are useful… in any application however, when your sample size is greatewr than 25 -30 statistically you can actually calculate the standrard deviation versus estimating as the Xbar chart does.
So use the Xbar & S when you have extremely large sample sizes.

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#173240

Bart
Participant

A nice definition from the Minitab help menu:
Xbar S process behaviors charts display a control chart for subgroup

means (an X chart) and a control chart for subgroup standard deviations (an S chart) in the same graph window. The X chart is drawn in the upper half of the screen; the S chart in the lower half. Seeing both charts together allows you to track both the process level and process variation at the same time, as well as detect the presence of special causes

. See [31] for a discussion of how to interpret joint patterns in the two charts.
X and S charts

are typically used to track process variation for samples of nine or more, while X and R charts

are used for smaller samples.
BDR

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