My histograms shows the data are rightly skewed.

So if the data are not normally distributed, i can't calculate the control limits or use the "tests for special causes", does it mean I have to convert non-normal data to normal ?

if so, how do I convert

So after conversion, how do I proceed from there? Will the data be "transformed" and I have to use the new figures?

Do I need to do "sub-grouping" ? If so, how would you suggest I group the data so that I can do Variable Charts for Subgroups > Xbar-S (in minitab) ?

Thanks !