Control charts for variables are classified according to the subgroup summary statistic plotted on the chart.
• XBar charts are a type of location chart that display subgroup means (averages).
• R charts are a type of dispersion chart that display subgroup ranges (maximum – minimum).
• S charts are a type of dispersion chart that display subgroup standard deviations.
• Presummarize charts display both subgroup means and standard deviations.
• Individual Measurement charts are a type of location chart that display individual measurements.
• Moving Range charts are a type of dispersion chart that display moving ranges of two successive measurements.
Note: If you remove a dispersion chart or turn off the preference Show Two Shewhart Charts in File > Preferences > Platforms > Control Chart Builder, you'll see only the location chart. Any associated scripts will contain the JSL option Show Two Shewhart Charts set to off (0).
For quality characteristics measured on a continuous scale, a typical analysis shows both the process mean and its variability with a mean chart aligned above its corresponding R or S chart.
Individual Measurement charts displays individual measurements. Individual Measurement charts are appropriate when only one measurement is available for each subgroup sample. If you are charting individual measurements, the individual measurement chart shows above its corresponding moving range chart. Moving Range charts displays moving ranges of two successive measurements.
If your data consist of repeated measurements of the same process unit, you can combine these into one measurement for the unit. Pre-summarizing is not recommended unless the data have repeated measurements on each process or measurement unit.
Presummarize summarizes the process column into sample means and/or standard deviations, based either on the sample size or sample label chosen. Then it charts the summarized data based on the options chosen in the window.
Levey-Jennings charts show a process mean with control limits based on a long-term sigma. The control limits are placed at 3s distance from the center line. The standard deviation, s, for the Levey-Jennings chart is calculated the same way standard deviation is in the Distribution platform.