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.MTW.
  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.

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

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

I-MR-R/S Standard Deviations of Parts

Standard Deviations Between 0.165737 Within 0.243244 Between/Within 0.294341 * WARNING * If graph is updated with new data, the results above may no longer be correct.
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