00:51 This chart relies on variable data, meaning data measured on a scale with.00:46 in some cases, even better results than the individual and moving range chart.00:42 And finally, even though it was less effort, it gave excellent results, and.00:39 control limits are very easy from a math perspective with this chart.00:32 a chart with only a sample of the data, rather than every single data point.00:29 calculations by hand, it took far less effort to create.00:25 Second, in the days when we had to do all these measurements and.00:21 And for a good reason, variable data is richer than attribute data for analysis.00:17 Historically, this has been the most commonly found SPC chart.00:12 > The X-bar and range or Xbar-R chart, is also a pair of charts.00:05 Let's now look at the most commonly found control chart, the Xbar-R chart.Don’t select arbitrary subgroups, they should relate to the physical process or calendar groupings.Be sure to use the subgroup sample size when selecting the A2, d3 and d4 constant values.Remember you are plotting the mean of the data, not the actual data.Take appropriate actions to remove special causes or to center your data within the customer spec limits.Plot the Subgroup mean, Range, the calculated Means and the control limits.Now calculate the control limits for the Subgroup Mean data points.If the Range plot shows an out of control condition, it must be corrected before proceeding to the Subgroup Mean plot, since that plot relies on the mean of the Range in its control limit calculations.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 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.
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