topic How to add control limits into a variability chart easily? in Discussions

How to add control limits into a variability chart easily?

Danial1 — Sun, 11 Jun 2023 11:08:42 GMT

I have a data table with multiple control limits for various parameters. How do I assign this control limits quickly into a variability chart? The method I'm currently doing is adding them manually into each variability charts then copy and save the scripts. Honestly I found it very time consuming and tiring. Could someone suggest an alternative. Sincerely appreciate it if there's a quicker way to work on it.

Re: How to add control limits into a variability chart easily?

statman — Fri, 04 Dec 2020 22:59:46 GMT

Hmmm...I don't want to sound unhelpful, but I don't understand why you would do this? Perhaps you could help educate me on the intended use?

While I think variability charts and control charts work well together, they are used for different purposes. The variability chart is a great way to look at data as it is simply a plot of the data. These are individual data points, not averages. Obvious patterns and even unusual data points can easily be seen.

Shewhart Control charts are meant to do two things:

1. The Range chart answers the question: Is the variability within subgroup stable, consistent?,

2. The X-bar chart answers the question: Which source has greater leverage, the within or the between subgroup

The control limits are a function of the within subgroup variation.

Now I could see possibly using limits based on the Moving Range, but, again those limits are a function of the surrogate subgroup of consecutive data points.

Re: How to add control limits into a variability chart easily?

txnelson — Sun, 06 Dec 2020 00:41:50 GMT

@statman questions and input is all quite valid, however, if you need to have the control limits added to your graphs, it isn't an overwhelming task to do so.

But, in order to guide you on how to accomplish this, I have a couple of questions.

Where are you getting the control limits from.
1. They are saved as control limit column properties?
2. They are in a separate data table?
3. They are on a slip of paper you have in one of your desk drawers?
4. Other
Do the control limits vary within a given Variability Chart?
About how many different charts do you have?