Example of U chart
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As production manager of a toy manufacturing company, you want to monitor the number of defects per unit of motorized toy cars. You inspect 20 units of toys and create a U chart to examine the number of defects in each unit of toys. You want the U chart to feature straight control limits, so you fix a subgroup size of 102 (the average number of toys per unit).

1    Open the worksheet TOYS.MTW.

2    Choose Stat > Control Charts > Attributes Charts > U.

3    In Variables, enter Defects.

4    In Subgroup sizes, enter Sample.

5    Click U Chart Options, then click the Limits tab.

6    Under When subgroup sizes are unequal, calculate control limits, choose Assuming all subgroups have size then enter 102.

7    Click OK in each dialog box.

Session window output

U Chart of Defects

 

 

 

Test Results for U Chart of Defects

 

 

TEST 1. One point more than 3.00 standard deviations from center line.

Test Failed at points:  5, 6

 

* WARNING * If graph is updated with new data, the results above may no

          * longer be correct.

Graph window output

 

Interpreting the results

Units 5 and 6 are above the upper control limit line, indicating that special causes may have affected the number of defects in these units. You should investigate what special causes may have influenced the out-of-control number of motorized toy car defects for these units.