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Sep 11, 2012

NP chart with variable sample sizes: what is JMP doing?

Both the JMP documentation and other references suggest an NP control chart of the number of defective units per sample should have a constant sample size. If I have a variable sample size, I should use a P chart of the proportion defective.

However, if I create an NP chart with a variable sample size, JMP creates a control chart with varying control limits and doesn't chastize me, at least not where I can see.

For example, starting from the Washers sample data set, I add a column titled "Lot Varies" with initial values random, normal distribution, mean = 400, standard deviation = 100, no decimal places. Then I launch Analyze >> Quality & Process >> Control Chart >> NP. In the launch dialog, # Defective is assigned to Process, Lot is assigned to Sample Label, and Lot Varies is assigned to Sample Size. JMP creates a perfectly OK-looking control chart.

Looking at the y-axis, it appears that the units are still in number of defective units, so JMP hasn't decided to develop a P chart instead just because I gave it a variable sample size. But JMP commonly makes reasonable assumptions about what to do based on the data supplied. What is JMP doing here?

Thanks for your assistance.