I have an sbar.

So I can calculate the UCL of an s-chart using B4 x sbar, and the LCL using B3 x sbar.

Now, using that same sbar, I can calculate UCL for an Xbar-S chart using Xdbar + A3 x sbar, and the LCL using Xdbar - A3 x Sbar.

OK.

Depending on where I look, sbar is called "standard deviation" or "sigma" (which I don't think is right), because..

There is another formula: UCL = Xdbar + 3 x sigma/sqrt

This one uses sigma not s, but it also is referring to standard deviation.

Can anyone help explain how to use sigma vice sbar, and when to use that Xdbar +3 x sigma/sqrt formula?

I appreciate your help.

Regards,

leaning