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About this lesson
This lesson discusses the unique considerations associated with monitoring attribute data with control charts. It compares and contrasts the various attribute data control charts and provides some ground rules for subgroups selection.
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Attribute Data Control Charts
There are four control charts that are normally used to monitor process attribute data. There are specific groundrules for determining which chart to select and the size of the subgroup to be used.
When to use
If a critical process output or process parament is attribute data – in particular pass/fail defect data – an attribute control chart should be used with the process.
Attribute data control charts are created using the control chart process discussed in an earlier module. The data on these charts will either be defects or defectives. The selection of which chart to use will depend upon whether charting defects or defectives and whether the sample or subgroup size being used is fixed or varies. The table below shows the differences between the chart types.
Unlike variable data control charts, the attribute control chart is a singular chart. There is no accompanying range chart.
The subgroup sizes for attribute charts are selected to allow an application of the central limit theorem in order to convert the pass/fail attribute data into a normal curve. Based upon this principle, the following ground rules should be followed.
- C charts should use subgroup sizes that create a mean of the subgroup that is greater than 2.
- U charts should use subgroup sizes so that there are at least five defects in each subgoup.
- P and NP should use subgroup sizes where the product of the subgroup size times the average of the subgroup proportions is greater than five, and the product of the subgroup size times the average of one minus the subgroup proportion is greater than five. This leads to a minimum value of 10 when the proportion is 50% and it will grow as the proportion average gets farther from that value.
Hints & tips
- You may need to try several different subgroup sizes until you find one that meets the ground rules.
- Although the four different types of charts plot attributes with different units, you still apply the control limits and special cause criteria in the same manner.
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