Variables Control Charts for Subgroups Overview
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Variables control charts for subgroups plot statistics from continuous measurement data, such as length or pressure, for subgroup data. Variables control charts for individuals, time-weighted charts, and multivariate charts also plot measurement data. Attributes control charts plot count data, such as the number of defects or defective units.

For more information about control charts, see Control Charts Overview.

Choosing a variables control chart for subgroups

Minitab offers five variables control charts:

·    X and R - an X chart and R chart displayed in one window

·    X and S - an X chart and S chart displayed in one window

·    I-MR-R/S (Between/Within) - a three-way control chart that uses both between-subgroup and within-subgroup variation. An I-MR-R/S chart consists of an I chart, a MR chart, and a R or S chart.   

·    X - a chart of subgroup means

·    R - a chart of subgroup ranges

·    S - a chart of subgroup standard deviations

·    Zone - a chart of the cumulative scores based on each point's distance from the center line

The I-MR-R/S (Between/Within) chart requires that you have two or more observations in at least one subgroup. Subgroups do not need to be the same size. Minitab calculates summary statistics for each subgroup, which are plotted on the chart and used to estimate process parameters.