Example of U Chart

A manager for a transcription company wants to assess the quality of the transcription service. The manager randomly selects 25 sets of pages from consecutive orders and counts the number of typographical errors (defects). Each set has a different number of pages.

The manager creates a U chart to monitor the number of errors.

  1. Open the sample data, TranscriptionErrors.MTW.
  2. Open the U Chart dialog box.
    • PC: STATISTICS > Control Charts > Attribute > U
    • Mac: Statistics > Attribute > U
  3. In Number of defects per unit column, enter Errors.
  4. From the drop-down list, choose Subgroup sizes are in a column. In Subgroup sizes column, enter Number of Pages.
  5. Click OK.

Interpret the results

Because the sample sizes are unequal, the control limits vary. The average number of defects per set of pages is 0.238. Subgroups 6 and 18 failed Test 1 because they are outside of the control limits. Thus, the process is out of control. The manager should identify and correct any factors that contribute to the special-cause variation.
Method
Parameter
Mean
Mean estimated
Tests for Special Causes
Results for U Chart
Test
Description
Failed Subgroups
1 point > 3 standard deviations from the center line
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