The XBar-R chart displays two charts, one for the Xbar values and the other for the Range values. The formulas for calculating the centerlines and control limits for the XBar-R control chart are detailed in the following sections.
XBar-R Chart Center Line
Based on the sample size, the mean for a given sample can be calculated as:
where x1 through xn are the values of each observation in the sample and n is the number of observations in the sample.
Because an XBar-R chart displays the average of each sample in the set, its centerline is calculated as:
where 
through 
are the averages of each sample,
and m is the number of
samples. This grand average becomes the centerline of the XBar
chart.
XBar-R Chart Control Limits
To construct the upper control limit (UCL) and lower control limit (LCL) for the XBar-R chart, we need to know the standard deviation of the data. This can be estimated with the range method.
In a sample of size n, the range is the difference between the smallest and largest observations. In other words:
If R1 through Rm is the range of m samples, then the average range is:
The formulas used for calculating the upper and lower control limits for the XBar chart are:
Where A2 is a tabulated value based on the sample size. See the XBar Chart Factors for Control Limits table for values of A2 for sample sizes ranging from 1 to 25.
If the sample size is greater than 25, the formulas are:
where S is the standard deviation for the chart.
R Chart Center Line
For the R chart, the formula for calculating the centerline is:
R Chart Control Limits
The formulas used for calculating the upper and lower control limits for the R chart are:
where D4 and D3 are tabulated values based on the sample size. See the Range Chart Factors for Control Limits table for values of D3 and D4 for sample sizes ranging from 1 to 25.
If the sample size is greater than 25, the formulas are:
where S is the standard deviation for the chart.
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XBar-R, XBar-S, R, S and X chart calculations. |
