Control Chart

Use control charts to indicate when a process is out of control and helps to identify the presence of special-cause variation.

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In This Topic

C chart
I-MR chart
NP chart
P chart
Run chart
U chart
Xbar-R/S chart

C chart

Use a C chart to monitor the number of defects where each item can have multiple defects. You should use a C chart only when your subgroup sizes are equal. If your subgroup sizes are not equal, use the U chart.

For example, a quality engineer for a wallpaper manufacturer wants to assess the stability of the printing process. Every hour, the engineer takes a sample of 100 feet of wallpaper and counts the number of printing defects, which include print smears, pattern distortions, and missing ink. To see an example, go to Minitab Help: Example of C Chart.

Data considerations

You must be able to count the number of defects on each item or unit. Subgroup sizes should be equal, or nearly equal. For details, go to Minitab Help: Data considerations for C Chart.

I-MR chart

Use an I-MR chart to monitor the mean and variation of your process when you have continuous data that are individual observations not in subgroups.

For example, a hospital administrator wants to determine whether the time to perform outpatient hernia surgery is stable and whether the variation in surgery times is stable. Because the data are not collected in subgroups, the administrator uses an I-MR chart to monitor the mean and variation of the surgery times. To see an example, go to Minitab Help: Example of I-MR Chart.

Data considerations

The data should be continuous and collected as individual observations, not subgroups. For details, go to Minitab Help: Data considerations for I-MR Chart.

NP chart

Use an NP chart to monitor the number of defective items where each item can be classified into one of two categories, such as pass or fail.

For example, a delivery service manager uses an NP chart to monitor the number of delivery vehicles that are out of service each day for 2 months. A vehicle that is out of service is considered a defective item. To see an example, go to Minitab Help: Example of NP Chart.

Data considerations

Collect data in subgroups. Items must be classified into one of two categories, such as pass or fail. For details, go to Minitab Help: Data considerations for NP Chart.

P chart

Use a P chart to monitor the proportion of defective items where each item can be classified into one of two categories, such as pass or fail.

For example, a delivery service manager uses a P chart to monitor the proportion of delivery vehicles that are out of service each day for 2 months. A vehicle that is out of service is considered a defective item. To see an example, go to Minitab Help: Example of P Chart.

Data considerations

Collect data in subgroups. Items must be classified into one of two categories, such as pass or fail. For details, go to Minitab Help: Data considerations for P Chart.

Run chart

Use a Run chart to look for patterns or trends in your data that indicate the presence of special-cause variation. A run chart plots your process data in the order that they were collected.

For example, you can use a control chart to verify that the process was stable. Then use a run chart to identify patterns that may not be detected in a control chart, but reveal clues for potential process improvements. To see an example, go to Minitab Help: Example of Run Chart.

Data considerations

Continuous or discrete data may be collected as individual observations or in subgroups. Collect enough data to represent the expected range of variation. The data must include at least 3 data points. However, you should collect enough data to represent patterns over the time period of interest. For details, go to Minitab Help: Data considerations for Run Chart.

U chart

Use a U chart to monitor the number of defects where each item can have multiple defects.

For example, an LCD manufacturer wants to monitor the number of dead pixels on 17-inch LCD screens. Technicians record the number of dead pixels for each screen. Each subgroup has a different number of screens. The manufacturer uses a U chart to monitor the average number of dead pixels per screen. To see an example, go to Minitab Help: Example of U Chart.

Data considerations

You must be able to count the number of defects on each item or unit. For details, go to Minitab Help: Data considerations for U Chart.

Xbar-R/S chart

An Xbar-R/S chart is a control chart that consists of two charts.

Use an Xbar-R chart to monitor the mean and variation of a process when you have continuous data and subgroup sizes of 8 or less. To see an example, go to Minitab Help: Example of Xbar-R Chart.

Use an Xbar-S chart to monitor the mean and variation of a process when you have continuous data and subgroup sizes of 9 or more. To see an example, go to Minitab Help: Example of Xbar-S Chart.

Data considerations

Your data must be continuous Y values collected in rational subgroups. For details, go to Minitab Help: Data considerations for Xbar-R Chart or Data considerations for Xbar-S Chart.