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There are three control charts that are normally used to monitor variable data in processes. Each chart has ground-rules 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 lesson. 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 or 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 defend 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 collection is manual, the collection can be expensive. In that case, you would prefer to just sample the process rather than doing 100% checking of the parameter. If the process uses batches, the sample should represent the batching. If the process is continuous flow, the subgroup will represent a portion of the flow. Normally, the subgroup is selected so that each sample represents approximately the same amount of flow. If the process is an infrequent process, the sample should represent that particular iteration of the process execution. If the data may contain some attributes that is non-normal, the sample can apply the Central Limit Theorem which creates a normal data set from non-normal data. It is helpful if the data is non-normal to use an odd number of data points in the sample so that there is a clear median value.
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.
- If the process runs at set times periods, let each time period be a subgroup.
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