vdoc

Chapter 19: B. Statistical Process Control 283
Continued
p Chart for Nonconforming Computer Chips with Variable Sample Sizes
0.018
0.016
UCL = 0.01675
0.014
Proportion
0.012
0.010
0.008
0.006
p – = 0.00813
0.004
0.002
0.000
3 6 9 12 15 18 21 24 27 30
Sample
LCL = 0
Figure 19.11
p chart for nonconforming chips with variable sample sizes, using trial
control limits for the data in Table 19.6.
The np Chart: Control Chart for Number of Nonconforming Units
In the np chart, we plot the number of nonconforming units in an inspected
sample instead of the fraction of nonconforming units in the inspected sample;
otherwise the np chart is very similar to the p chart. However, note that in the
p chart the sample sizes could be equal or unequal, whereas in the np chart
the sample sizes are equal. Otherwise, the p chart and np chart can be implemented
under the same circumstances. Following are some specific points that
are pertinent for np charts:
• The inspection sample sizes should be equal.
• The sample size should be large enough to include some
nonconforming units.
• Record the sample size and number of observed nonconforming
in each sample, and plot the number of nonconforming on the
control chart.
Part IV.B.4
Control Limits for np Control Chart
Select m samples each of size n from the process under investigation and then
determine the number of nonconforming units that exist in each sample. Let the
number of nonconforming units found be denoted by x 1 , x 2 , x 3 , . . . , x m respectively.
Then the control limits are found as follows.