Example of I-MR-R/S Chart

A quality engineer for a plastic parts company monitors an injection molding process. A machine has a die that creates five parts at one time. The engineer collects 20 subgroups of five parts each. The engineer monitors both the within-subgroup variation between parts that are created at the same time and the between-subgroup variation between the batches.

The engineer creates an I-MR-R/S chart to monitor the injection molding process.

  1. Open the sample data, InjectionMolding.MWX.
  2. Choose Stat > Control Charts > Variables Charts for Subgroups > I-MR-R/S (Between/Within).
  3. From the drop-down list, select All observations for a chart are in one column, enter Parts.
  4. In Subgroup sizes, enter Subgroup.
  5. Click I-MR-R/S Options.
  6. On the Tests tab, select 1 point > K standard deviations from center line (Test 1) and K points in a row on same side of center line (Test 2).
    If you are not sure which tests apply in your specific situation, use Tests 1 and 2.
  7. Click OK in each dialog box.

Interpret the results

One subgroup failed Test 1 on the Moving Range chart, and one point failed Test 1 on the R chart. The between-subgroup variation and the within-subgroup variation are not in control. No subgroups failed the test for special causes on the I chart. The I chart shows no evidence of lack of control in the process location.

Test Results for MR Chart of Subgroup Means of Parts

TEST 1. One point more than 3.00 standard deviations from center line.
Test Failed at points:  14

Test Results for R Chart of Parts

TEST 1. One point more than 3.00 standard deviations from center line.
Test Failed at points:  8

Standard Deviations

Between0.165737
Within0.243244
Between/Within0.294341