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SPC Control Charts ​

Easson2D SPC provides several control charts for observing whether the measurement data remains stable during continuous production.

The following are supported:

  • XBar-R
  • XMed-R
  • XBar-S
  • X-Rm

Before use, first select the control item to analyze in the lower part of SPC Manager, then click the corresponding control chart button.

XBar-R Control Chart ​

The XBar-R chart consists of two charts:

  • Upper chart: the mean of each group (XBar)
  • Lower chart: the range of each group (R)

It can simultaneously observe whether:

the central position of each group changes, and whether the fluctuation of each group is stable.

For example, when the sample size is set to 5, every 5 consecutive samples form a group, and the mean and range of each group are calculated separately.

XMed-R Control Chart ​

The XMed-R chart also consists of two charts:

  • Upper chart: the median of each group (XMed)
  • Lower chart: the range of each group (R)

It is similar to XBar-R, but uses the median to represent the central position of each group.

XBar-S Control Chart ​

The XBar-S chart consists of:

  • Upper chart: the mean of each group (XBar)
  • Lower chart: the standard deviation of each group (S)

Unlike XBar-R, which uses the range to observe fluctuation, XBar-S uses the standard deviation to reflect the dispersion of each group.

X-Rm Control Chart ​

The X-Rm chart is an individual values control chart.

It consists of:

  • Upper chart: the actual measured value of each sample (X)
  • Lower chart: the moving range between adjacent samples (Rm)

X-Rm analyzes each sample directly, so unlike XBar-R, XMed-R, and XBar-S, it does not rely on the "sample size" for grouping.

This can be simply understood as:

The first three control charts look at "each group", while X-Rm looks at "each sample".

UCL, CL, and LCL ​

Control charts usually display three reference lines:

  • UCL: upper control limit
  • CL: center line
  • LCL: lower control limit

When a data point exceeds the control range, the software highlights it with a different color to help you find positions where abnormal variation may have occurred.

Note that:

UCL / LCL are control limits calculated from the process data, not the product specification limits.

The specification limits are mainly used to judge whether a product is out of spec, while the control limits are mainly used to observe whether the production process is stable.

Simple Differences among the Four Control Charts ​

Control ChartMain observation content
XBar-RGroup mean + range
XMed-RGroup median + range
XBar-SGroup mean + standard deviation
X-RmIndividual sample value + moving range

For general grouped measurement, you can use XBar-R, XMed-R, or XBar-S as needed.

If you want to directly observe data variation sample by sample in a continuous series, you can use X-Rm.

Viewing the Control Charts ​

The basic workflow is:

Select the control item → Set a suitable sample size → Select the control chart type → View UCL / CL / LCL and data point variation

Through the control charts, you can observe whether abnormal fluctuation occurs in the production process more intuitively than by only looking at the measured values.