The Xbar Chart is a control chart of subgroup means. It is used to track the process level and detect signs of special causes. 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 > Xbar
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 Mean, enter the population proportion, otherwise it will be estimated from the sample (Optional, Numeric, >0 & <1)
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 Xbar Chart to track the process level.
Open data worksheet by choosing ProcessMA > Tools > Data
Choose ProcessMA > Control Charts > Variables Charts for Subgroups > Xbar
In Variable, select V - Waiting Time
In Constant Subgroup Size, enter 10
Click OK
The fifth point lies outside of the control limit suggesting a possible special cause.
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