Example of Individuals 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 Individuals chart to monitor the detergent process.

  1. Open the sample data, DetergentpH.MTW.
  2. Choose Stat > Control Charts > Variables Charts for Individuals > Individuals.
  3. In Variables, enter pH.
  4. Click I Chart 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 (Test 2).
    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

One observation, failed Test 1 on the individuals chart (I chart) because the observation is more than 3 standard deviations above the center line. The engineer concludes that the pH level of the detergent is not stable.

I 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.