(I know it is not quite appropriate to quote it as "part 2", but since the question has been dormant for quite a while, I hope by doing this will rise peoples' attention again, you may have a look of part I here.)
I have come across an article online talking about the case similar to mine, that most of the time the case count is zero, make sometimes when the case number increases to one, it already shoots above the control level and consider the case as "out of control".
Since c-chart will be easier for my bosses to read and interpret, I wonder if the method is sound, or did anyone have some more official reference on this method? (I have googled quite a while but I can find nothing)
The article can be found here.
An to further my question, I want to ask one more thing: for the assumption of c-chart that the case count needs to follow a Poisson distribution, is it applicable to all lambda (i.e. mean of case count)?