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About this lesson
The Individual and Moving Range chart is the simplest of the variable data control chart. This lesson explains how the data is recorded and interpreted on the chart. The lesson describes how to create this control chart in both Microsoft Excel and using Minitab. The lesson will include practice creating the chart.
The I-MR chart (Individuals and Moving Range) is the variable data control chart to be used when each item is processed uniquely and therefore a subgroup sample size of one is appropriate. It can be easily created in either Microsoft Excel or Minitab.
When to use
Use the I-MR chart when seeking to control a product or process variable parameter. It is often used with critical characteristics that are identified on design documentation. It is also frequently used with low volume processes that complete one item at a time.
The I-MR is comprised of a pair of control charts. These are normally aligned so that the Individual chart is directly above the Moving Range chart. This aids in the recognition of special cause occurrences. The Individual chart plots the data value as measured. The Moving Range chart plots the absolute value of the difference between the current data point and the previous data point. (For this reason, there is no Moving Range value for the first point since there is no pervious data point.
I-MR Charts can be created in Microsoft Excel or in Minitab. 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 Individuals” and then finally select “I-MR.” In the Minitab I-MR Chart panel, you will need to select the data column with your data.
If creating the I-MR Chart in Excel:
- Measure the attribute and record the data in a column in Excel.
- In an adjacent column, calculate the Moving Range for each data point.
- Calculate the Mean for the Moving Range data and the Individuals data.
- Calculate the upper control limits for the Moving Range plot. The lower control limit is always zero for this chart.
- If the Moving Range plot shows an out of control condition, it must be corrected before proceeding to the Individual plot, since that plot relies on the mean of the Moving Range in its control limit calculations.
- Now calculate the control limits for the Individual data points.
- Plot the data points, the Mean and the control limits.
- Take appropriate actions to remove special causes or to center your data within the customer spec limits.
Hints & tips
- The LCL for the Moving Range is always zero.
- Be sure to use the subgroup sample size of 1 when selecting the d2 and d4 constant values.
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