Example of NP chart
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You work in a toy manufacturing company and your job is to inspect the number of defective bicycle tires. You inspect 200 samples in each lot and then decide to create an NP chart to monitor the number of defectives. To make the NP chart easier to present at the next staff meeting, you decide to split the chart by every 10 inspection lots.

1    Open the worksheet TOYS.MTW.

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

3    In Variables, enter Rejects.

4    In Subgroup sizes, enter Inspected.

5    Click NP Chart Options, then click the Display tab.

6    Under Split chart into a series of segments for display purposes, choose Each segment contains __ subgroups and enter10.

7    Click OK in each dialog box.

Session window output

NP Chart of Rejects

 

 

 

Test Results for NP Chart of Rejects

 

 

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

Test Failed at points:  9, 20

 

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

          * longer be correct.

Graph window output

 

Interpreting the results

Inspection lots 9 and 20 fall above the upper control limit, indicating that special causes may have affected the number of defectives for these lots. You should investigate what special causes may have influenced the out-of-control number of bicycle tire defectives for inspection lots 9 and 20.