Hi, everyone!
What is the formula for calculating sigma for Cp, Cpk? X-bar chart. (Short term)
The answer is provided in the JMP Help. See this page.
Its based on the moving range
http://www.jmp.com/support/help/en/15.1/#page/jmp/control-limits-for-individual-measurement-and-moving-range-charts.shtml#ww1157183