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

Test for Special Causes Results - R Chart

  

The R chart assesses whether the within-subgroup variation is in control. Each test for special causes detects points beyond the control limits and specific patterns in the data on the moving range (MR) chart.

·    When a subgroup fails a test, Minitab reports it in the Session window and marks it on the R 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.

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.

Example Output

Test Results for R Chart

TEST 1. One point more than 3.00 standard deviations from center line.

Test Failed at points: 8 

Interpretation

For the injection molding data, subgroup 8 failed Test 1 on the R chart. The failed subgroup indicates that its variation is larger than expected. Test 1 tests for subgroup that are more than 3 s from the center line and provides the strongest evidence for lack of control in a process.