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

Test for Special Causes Results - S Chart

The S chart assesses whether the within-subgroup variation is in control. Each test for special causes detects a specific pattern in the data plotted on the S chart.

· When a subgroup fails a test, Minitab reports it in the Session window and marks it on the S chart. A failed subgroup indicates that there is a nonrandom pattern in the data that may be the result of special-cause variation. These subgroups should be investigated.

· When no points are displayed in the Session window, then no subgroups failed the tests for special causes.

For the injection molding data, the subgroup size is five; therefore no S chart is displayed.

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.