Example of I-MR Chart

A quality engineer monitors the manufacture of liquid detergent and wants to assess whether the process is in control. The engineer measures the pH of 25 consecutive batches of detergent.

The engineer creates an I-MR chart to monitor the detergent process.

  1. Open the sample data, DetergentpH.MTW.
  2. Choose Stat > Control Charts > Variables Charts for Individuals > I-MR.
  3. In Variables, enter pH.
  4. Click I-MR Options.
  5. 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.
    If you are not sure which tests apply in your specific situation, use Tests 1 and 2.
  6. Click OK in each dialog box.

Interpret the results

Interpret the Moving Range chart (MR chart) first to examine the process variation. None of the points are outside the control limits and the points display a random pattern. Thus, the process variation is in control, and the quality engineer can examine the process center on the Individuals chart (I chart).

One observation failed Test 1 on the I chart because the observation is more than 3 standard deviations above the center line.

I-MR Chart of pH

Test Results for I Chart of pH

TEST 1. One point more than 3.00 standard deviations from center line. Test Failed at points: 8 * WARNING * If graph is updated with new data, the results above may no longer be correct.
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