P chart vs. IMR chart

Is the data you are plotting binomial?

As usual, you are getting erroneous information. Yes, Wheeler says you can switch to an I/MR chart if the p>=5. If your sample size is getting too large, you start to violate some of the assumptions of the binomial and the large n in the denominator are giving you a false tightening of the control limits. If p>=5 then an assumption of continuous/normal can be made and thus the use of the I/MR.

Darth,
Do you mean np? 
Darth stated the sample size concern better than I did, sorry about that.

Hi JanI’ve been involved in some charting of data on the process of care of stroke patients. It was a state requirement that monthly proportions of patients having a CT brain scan within 48 of admission to hospital be reported. We decided not to use p-charts for this data because the probability of having a scan within 48 hours is not constant from patient to patient since, for example, the provision of radiographers differs (sadly) between week-ends and week days. Thus we could not assume a binomial distribution which underpins the p-chart. We therefore used an Individuals (X)chart. Apparently justification for this approach may be found in the Western Electric Quality Control Handbook from 1956.You might find it of interest to read Improved Control Charts for Attributes by David Laney available in full and free at http://www.quadrantonegroup.com/pdf/laneydoc.pdf. He introduces the p’ chart and argues it does a better job than the X chart when binomial assumptions are not met. I’ve written a crude Mintab macro to produce a basic p’chart which I’ll happily pass on to you if you contact me at [email protected] WishesBower Chiel

Jan,
Dr. Donald Wheeler as always is  a great mentor I highly recommend his book “Understanding Statistical Process Control”.
P charts are not the chart of choice in most cases.