12.3. CHECKING FOR A CHANGE BETWEEN TWO SAMPLES
We sometimes want to compare samples from two similar processes or from one process at different times. As in comparing a sample with a population, we do three steps in checking for a change between two samples.
Check the distributions to see if they are substantially different.
If the distribution shapes are not substantially different, then see if the sigmas are significantly different.
If neither of the above tests shows a difference, then check if the averages are significantly different.
If we see a difference at any of the above steps, it is important to know, since it affects costs, quality, etc.
1. Checking the Distributions
First, it may be necessary to plot a large number of individual ...
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