topic Re: How do fix the warning "Asymmetric control limit" in the SPC chart? in Statistical Procedures

How do fix the warning "Asymmetric control limit" in the SPC chart?

bhr-q — Sun, 09 Jun 2024 21:03:46 GMT

when I run the below code to perform the special cause test, I receive the warning shown below. Could you please advise on how to fix that?

proc shewhart data=tmp10;
pchart yes_answer*month /Markers subgroupn =  n_per_group     ODSTITLE="P-chart with Tests for Special Causes" 
                           tests = 1 to 4
                           testnmethod = standardize
                           table
                           tablelegend
                           nohlabel;
 LABEL yes_answer ="Prop of Pts Receiving IV Abx";                          
run;

WARNING: Asymmetric control limits encountered for yes_answer for at least one subgroup.
WARNING: It is assumed that the difference between the upper limit and center line represents three standard errors.

Thank you!

Re: How do fix the warning "Asymmetric control limit" in the SPC chart?

Quentin — Sun, 09 Jun 2024 21:42:35 GMT

Asymmetric control limits often happen for a p-chart, because the lower control limit is truncated at 0. The warning says "It is assumed that the difference between the upper limit and center line represents three standard errors." I would think that's a fair assumption, I don't see anywhere in your code that asked for different control limits. On the chart, it shows the upper control limit as a 3-sigma limit, right? I don't think I'd worry about this warning.

Re: How do fix the warning "Asymmetric control limit" in the SPC chart?

bhr-q — Sun, 09 Jun 2024 22:13:19 GMT

Thank you for your answer, I have attached the chart, which indicates that the UCL is set at a 3-sigma limit. However, the LCL does not show a corresponding 3-sigma limit. I think this discrepancy is due to varying sample sizes and the proportion being very high for each subgroup, with the UCL approaching one. (in my case LCL doesn't truncate at zero)

Re: How do fix the warning "Asymmetric control limit" in the SPC chart?

Quentin — Mon, 10 Jun 2024 01:20:37 GMT

Agree, you've got 3-sigma limits. But in your case the UCL is truncated by the maximum value, 1. I wouldn't worry about the warning message. Looking at tests 1-4, I think they should be fine.