Thank you for this answer; I noticed that the number of data points within the P-chart should be at least 20, so I decided to create the P-chart by month, not by quarter, as above.
when I created the P-chart by month, the sample size of each subgroup was as below; I am not sure if I am eligible to use the P-chart for this sample size. Or is the P-chart reliable for this sample size? I found somewhere the number of sample size for each subgroup to use the P chart is at least 50, not sure, I guess you might know the answer
10
19
20
24
24
20
24
12
15
23
30
20
24
27
21
32
23
24
26
35
27
33
38
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