The S Chart is a control chart of subgroup standard deviations. It is used to track the process variation and detect signs of special causes. It is suitable for sample sizes larger than 8. For smaller sample sizes, the R Chart may be more appropriate. The process standard deviation is estimated using pooled standard deviation as default.
At the Excel Menu (For Excel 2007, go to Add-ins first)
Choose ProcessMA > Control Charts > Variables Charts for Subgroups > S
In Variable, select the column containing the data (Numeric)
In Subgroup, select the column containing subgroup indicators (Optional, >=2 distinct values)
In Constant Subgroup Size, enter the subgroup size if it is constant (Optional, Integer, >=2)
In Historical Standard Deviation, enter the population standard deviation, otherwise it will be estimated from the data (Optional, Numeric, >0)
In Control Limit Multiples, enter the position to draw the upper and lower control limits. Multiple entries must be separated by commas (Optional, Numeric, >0, Up to 3 multiples)
In Lower Limit Bound, enter the bound on the lower control limit (Optional, Numeric)
In Upper Limit Bound, enter the bound on the upper control limit (Optional, Numeric)
In Phase, select the column containing the phase indicators (Optional, >=2 distinct values)
In Method, choose the method of estimating sigma
Check the various checkboxes to perform respective tests
Click OK
You are the branch manager of a bank and you need to ensure that time customers spent waiting in the queue is in control. For each day you took 10 measurements on the waiting time. You want to generate a S Chart to track the variation.
Open data worksheet by choosing ProcessMA > Tools > Data
Choose ProcessMA > Control Charts > Variables Charts for Subgroups > S
In Variable, select V - Waiting Time
In Constant Subgroup Size, enter 10
Click OK
The process is stable as the data points are randomly distributed and lie within the control limits.
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