Shewhart variables control charts

Variables control charts plot quality characteristics that are numerical (for example, weight, the diameter of a bearing, or temperature of the furnace).

There are two types of variables control charts: charts for data collected in subgroups, and charts for individual measurements. For sub-grouped data, the points represent a statistic of subgroups such as the mean, range, or standard deviation. For individual measurements, the points represent the individual observations or a statistic such as the moving range between successive observations.

An important concept for data collected in subgroups is that of a rational subgroup. A rational subgroup is a number of units produced under the same set of conditions. The measurements in each subgroup should be taken within a short period and should minimize the chances of special-cause variation occurring between the observations. That is, it should represent a snapshot of the process and the inherent common-cause variation.

Subgroups are often preferable when learning about a process as they can better estimate the process variability or detect smaller shifts in the process average. However, when monitoring a process, if the amount of time taken to collect enough observations to form a subgroup is too long, or the cost of sampling is expensive or destructive then the benefits may be negated.

• Xbar-R chart
  An Xbar-R chart is a combination of control charts used to monitor the process variability (as the range) and average (as the mean) when measuring subgroups at regular intervals from a process.
• Xbar-S chart
  An Xbar-S chart is a combination of control charts used to monitor the process variability (as the standard deviation) and average (as the mean) when measuring subgroups at regular intervals from a process.
• Xbar chart
  An Xbar-chart is a type of control chart used to monitor the process mean when measuring subgroups at regular intervals from a process.
• R chart
  An R-chart is a type of control chart used to monitor the process variability (as the range) when measuring small subgroups (n ≤ 10) at regular intervals from a process.
• S chart
  An S-chart is a type of control chart used to monitor the process variability (as the standard deviation) when measuring subgroups (n ≥ 5) at regular intervals from a process.
• I-MR chart
  An I-MR chart is a combination of control charts used to monitor the process variability (as the moving range between successive observations) and average (as the mean) when measuring individuals at regular intervals from a process.
• I chart
  An I-chart is a type of control chart used to monitor the process mean when measuring individuals at regular intervals from a process.
• MR chart
  An MR-chart is a type of control chart used to process variability (as the moving range of successive observations) when measuring individuals at regular intervals from a process.