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Variable Data Control Charts
There are three control charts that are normally used to monitor variable data in processes. Each chart has groundrules for the subgroup size and differences in how the control limits are calculated.
When to use
If the critical product or process parameter being monitored is measured using variable data measurement techniques, that a variable data SPC control chart should be used for tracking and controlling that parameter.
Variable data control charts are created using the control chart process discussed in an earlier module. The data on these charts is measured data. These control charts are always shown in pairs with one chart plotting the data value or a representative of the data value and the other chart plotting a measurement that represents the variation of dispersion of the data in the subgroup. The control charts will follow the typical pattern of a time-based plot of sequential data points, with a mean value line and both upper control limits and lower control limits.
The selection of which chart to use will depend upon the size of the data sample in the subgroup. When the subgroup sample size is a single data point, use the I-MR charts. When the subgroup sample size is two to ten data points, use the XbarR charts. When the subgroup sample size is greater than ten data points, use the XbarS charts. The size of the subgroup sample is based upon several factors. If the data is not normal, a sample is needed to apply the Central Limit Theorem.
A common method for determining how to set a subgroup sample size is based upon some physical or time-based limitation to the product or process. This would include using a batch process to define the subgroup and data samples within the batch or using a time increment, such as daily, to define the subgroup and then taking data samples every hour during the day. When Shewhart developed these control charts, he was using three standard deviations as his guide for control limits, but the statistical derivations had to bring into consideration the uncertainty of small sample sizes.
Ultimately, a set of tables with constants was created and these are used in the calculations of control limits. These tables are presented here, because some of these constant values are used with multiple variable data control charts. Notice that the first column in both tables is the subgroup sample size.
Hints & tips
- Use a subgroup size that makes sense for how the process works. If each item is being uniquely processed, use the I-MR..
- If the process works in batches use the batch as the subgroup.
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