I-MR-R/S (Between/Within) Chart

Summary

  

Use I-MR-R/S (Between/Within) chart to create a control chart that uses both between-subgroup and within-subgroup variation. For example, a process for coating paper with a thin film may have both between-subgroup and within-subgroup variation. The within-subgroup variation is the variation is film thickness within one roll (subgroup). If the machine is reset for each roll, then there might be variation between rolls (subgroups).

The I-MR-R/S chart consists of the following:

·    Individuals (I) chart, which charts subgroup means and can be used to assess process location.

·    Moving Range (MR) chart, which charts the between-subgroup component of variation.

·    R chart or S chart, which charts the within-subgroup component of process variation.

Note

Whether Minitab displays an R chart or an S chart depends on the size of the subgroups. If more than 50% of the subgroups are the same size, then the most common subgroup size determines the chart. Otherwise, the largest subgroup size determines the chart. If the subgroup size is 8 or less, Minitab displays an R chart. If the subgroup size is 9 or more, Minitab displays an S chart.

The combination of the three charts provides a method of assessing the stability of process location, the between-subgroup component of variation, and the within-subgroup component of variation.

Data Description

An injection-molding company uses a die to create parts. Each die produces five parts at one time. Twenty subgroups of five parts each were collected.

Data: InjectionMolding.MTW (available in the Sample Data folder).