The three most common types of SPC control charts for variables data are the Xbar and Range (Xbar-R) chart, used when small subgroups are sampled at regular intervals; the Individual-X Moving Range (X-MR) chart, used when only one reading is taken per sample; and the Xbar and Standard Deviation (Xbar-S) chart, used to monitor variation within larger subgroups.
If you have decided to use statistical process control (SPC), the control chart is the tool that tracks a process over time and flags abnormal variation.
So, what’s the next step?
Determining which type of control chart to use.
“That decision is based on the style of the data stream that you would like to be presented,” Steve Wise, vice president of Statistical Methods for InfinityQS, says. “Trained statisticians will know when they walk up to a data stream what core control chart they should start with, but the first step is to ask yourself what decisions you want to make from this chart. That will help you dictate the chart type and sampling strategies.”
Control charts typically fall into three categories. Let’s take a quick look at the different types of control charts here. For a deeper dive, visit our Definitive Guide to SPC Charts.
1. Xbar and Range (Xbar-R) Chart
The most common type of SPC chart for operators searching for statistical process control, the Xbar and Range chart, is used to monitor a variable’s data when samples are collected at regular intervals. The chart is particularly advantageous when your sample size is relatively small and constant. In practice that means subgroups of two to nine readings taken at each interval.
2. Individual-X Moving Range (X-MR) Chart
When it doesn’t make sense to take multiple readings, the Individual-X Moving Range chart is the ideal option. This particular SPC chart is used to monitor variables data when it is impractical to use rational subgroups. “When data is very expensive or there is a whole lot of time between samples, the concept of Xbar and Range makes no sense,” Wise explains. “It is better to go with Individual-X Moving Range.”
3. Xbar and Standard Deviation (Xbar-S) Chart
This chart monitors the subgroup average alongside the subgroup standard deviation, so it shows how much variation or dispersion exists around the expected value. Use it instead of Xbar and Range when subgroups are larger (typically 10 or more readings), because standard deviation estimates within-subgroup variation more accurately than the range at that size.
“The more you peel back as you start exploring the data stream, the more you need to refine the chart type you select,” says Wise.
Which Type of Control Chart Should You Use?
- One reading per sample, or long gaps between samples: Individual-X Moving Range (X-MR)
- Subgroups of two to nine readings at regular intervals: Xbar and Range (Xbar-R)
- Subgroups of ten or more readings: Xbar and Standard Deviation (Xbar-S)
- Counts of defects or defectives rather than measurements: attributes charts (p, np, c, u), covered in the SPC chart guide
Statistical process control software selects the chart from the sampling plan and recalculates limits as data arrives. For the full set, see the SPC chart guide. InfinityQS and WinSPC build Xbar-R, X-MR and Xbar-S charts automatically from shop-floor data. Get a demo.
