topic Re: Control chart: Xbar and S chart with large subgroup size unable to extract or calculate limits in Discussions

Control chart: Xbar and S chart with large subgroup size unable to extract or calculate limits

Abby_Collins14 — Thu, 08 Aug 2024 21:36:38 GMT

I am working on building Xbar and S chart to monitor a process where we observe the data (Y) every 1 minute across multiple runs. I wanted a chart where I have the control limits be the average of each time point across runs, which I believe the Xbar and S chart is doing. However, the actual LCL and UCL limits are not showing up, and I don't know how to extract them from the plot (see example chart below). Does anyone have suggestions for how to do this?

Thanks!

Re: Control chart: Xbar and S chart with large subgroup size unable to extract or calculate limits

shampton82 — Fri, 09 Aug 2024 01:58:47 GMT

Hye @Abby_Collins14 ,

Since your control limits are moving you it means you don't have equal sample sizes and thus JMP doesn't know what Control Limit to report. You might need to change your data pull to make sure the subgroups have the same sample size. You could also just run the analysis where you have the same sample sizes and get the control limits from there.

Steve

Re: Control chart: Xbar and S chart with large subgroup size unable to extract or calculate limits

Abby_Collins14 — Fri, 09 Aug 2024 14:11:45 GMT

Thanks! Is there a way to pull the moving control limits? That is what I am after

Re: Control chart: Xbar and S chart with large subgroup size unable to extract or calculate limits

Byron_JMP — Fri, 09 Aug 2024 18:40:26 GMT

Since you aren't really using a control chart, but a Level Jennings chart which uses the overall variation of the subgroup rather than the between subgroup, you could just use the tabulate function to calculate the standard deviation for each subgroup and go from there.

BTW, using standard deviation control limits is a good way to Never see anything out of control

Re: Control chart: Xbar and S chart with large subgroup size unable to extract or calculate limits

shampton82 — Sat, 10 Aug 2024 15:54:00 GMT

I think this will get you what you are after