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280 Part IV: Quality Assurance
Continued
p Chart for Nonconforming Computer Chips
0.018
0.016
UCL = 0.01701
0.014
Proportion
0.012
0.010
0.008
0.006
p – = 0.00837
0.004
0.002
0.000
3 6 9 12 15 18 21 24 27 30
Sample
LCL = 0
Figure 19.10
p chart for nonconforming computer chips, using trial control limits for
the data in Table 19.5.
conditions on these days should be made so that similar conditions can be replicated
for future use. Otherwise, since all the points of the current data fall within the control
limits, the trial control limits can be extended for use over the next 30 days, when the
control chart should again be reevaluated.
Part IV.B.4
The p Chart: Control Chart for Fraction Nonconforming with
Variable Samples
There are times when, for various reasons, it is not possible to select samples
of equal size. This is particularly true when the samples consist of 100 percent
inspection during a fixed period of time on each day of the study. The procedure
to develop a p chart with variable sample sizes is very similar to the procedure for
a p chart with constant sample size.
Thus, for example, suppose we have m samples of sizes n 1 , n 2 , n 3 , . . . , n m . Then
to develop a p chart for variable sample sizes we proceed as follows:
1. From the process under investigation select m (m 25) samples of sizes
n 1 , n 2 , n 3 , . . . , n m (n i 50) units.
2. Find the number of nonconforming units in each sample.
3. Find for each sample the fraction p i of nonconforming units