The lower control limit is zero for this chart if the number of data points in the subgroup is seven or less. Calculate the upper and lower control limits for the Range plot.Calculate the global mean for all the data points and the mean for the Range values.These will be the values that are plotted. In an adjacent column, calculate the Mean for each subgroup and then calculate the range from the smallest to the largest data point in the subgroup.By doing this, each row in Excel represents a subgroup. Then measure the next item in the subgroup sample and record that in the next column. Measure the attribute for the first item in the subgroup sample and record the data in a column in Excel.If all the data is in one column, you will need to tell Minitab how many data points are in each subgroup. Within Minitab, control charts are created by using the “Stat” pull down menu, then selecting “Control Charts.” Within the Control Charts window, select “Variable Charts for Subgroups” and then finally select “Xbar-R.” In the Minitab Xbar-R Chart panel, you will need to select the data columns with your data. Xbar-R Charts can be created in Microsoft Excel or in Minitab. Therefore the Range value is always a positive number – even if all the data values were negative numbers. The Range chart plots the value of the difference between the largest point and the smallest point in the subgroup data points. The Subgroup Mean chart plots the average value of the data points that were used to sample the subgroup. This aids in the recognition of special cause occurrences. These are normally aligned so that the Subgroup Mean (Xbar) chart is directly above the Range chart. The Xbar-R is comprised of a pair of control charts. And it is quite easy to do the math associated with calculating the control limits. It is more sensitive to shifts in the mean than the I-MR chart. It can be used with both high volume and low volume processes. It is the most commonly used variable data control chart. Use the Xbar-R Chart when seeking to control a product or process variable parameter. It can be easily created in either Microsoft Excel or Minitab. ![]() The collective effort can lead to savings and better throughput.The Xbar-R chart (Subgroup Mean and Range) are the variable data control chart to be used when working with a subgroup sample of two to ten units. This will help to boost continuous flow of materials, increase equipment uptime and reduce changeover time. Manufacturers should consider implementing these charts with the addition of lean tools, techniques and methods.
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