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. Open the I-MR Chart dialog box.
    • PC: STATISTICS > Control Charts > Continuous > I-MR
    • Mac: Statistics > Continuous > I-MR
  3. In Variables, enter pH.
  4. Click OK.

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.

Method
Parameters
Mean
StDev
Both parameters estimated
Tests for Special Causes
Results for I Chart
Test
Description
Failed Observations
1 point > 3 standard deviations from the center line
8
Results for Moving Range Chart
Test
Description
Failed Observations
1 point > 3 standard deviations from the center line
None
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