Tips & Tricks

Choosing the right variable control chart for your process

Did you know that there are three types of variable control charts you may use to analyze your data? By choosing a chart based on your sample size, you gain a more accurate picture of your process. So, before you use the Xbar-R chart again, stop to consider your collected data set. There may be a better variable control chart for you to use.

Choosing the correct type of control chart based on your sample size provides a clear picture of your process, allows the chart to quickly reflect process change, and offers just the right amount of analysis.

The three types of variable control charts to choose from are Xbar-R, Xbar-S, and X-MR.

Xbar-R charts plot the range by taking the difference of the maximum and minimum values for each subgroup. This calculation is most beneficial when your subgroup size is relatively small. As a guideline, this chart is typically effective for subgroup sizes containing two to 10 measurements.

If your collected data set is large, calculating a range using just maximum and minimum values could leave data unanalyzed. While the Xbar-R chart is the most commonly used chart, the Xbar-S chart is effective if you'd like to understand what is happening between the maximum and minimum values.

Instead of plotting the range, Xbar-S charts plot the standard deviation of the measurements in a subgroup. As a guideline, these charts are usually used for subgroups containing more than 10 measurements. When plotting the standard deviation, this chart uses an equation which takes into account every measurement in the subgroup, not just the minimum and maximum values. This provides better insight to your process by eliminating gaps.

So, what if your collected data set is equal to one? The X-MR chart is used specifically for this situation.

Similar to the Xbar-R chart, X-MR charts, also called Individuals-Moving Range charts, plot the range. In this case the range is a moving range--or MR. Rather than calculating a range using maximum and minimum values within a subgroup, the MR is calculated by taking the difference of every two consecutive readings. The example below illustrates this process.

Lot X MR
1 6.8 -
2 6.9 |6.9-6.8|=0.1
3 6.6 |6.6-6.9|=0.3
4 6.7 |6.7-6.6|=0.1 and so on...

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