I have an Excel worksheet from someone smarter than me and I am trying to understand what mathematic process is being calculated.
Cell F9 contains $$ =\mathop{CHISQ.INV.RT}(F8, 2)$$ where F8$=0.0063%$ (this is labeled "Fraction defective").
I have gathered that this comes from $C_p/C_{pk}$ of $1.33\dot{3}$ which is a 4-sigma process and which is 63.342 defective parts per million. This much makes sense knowing the context of this excel sheet.
Where I am stuck is the formulas in cells F11, F12, and F13 (labeled xl, xu, and xu$-$xl). I assume this is x lower, x upper and the difference between them and think that this is some sort of confidence interval. However, I cannot find a formula online anywhere that matches the formula they used.
In F11 they have: F4-F5$\ast SQRT($F9$)+0.00001$ where F4 is the average and F5 is the standard deviation. Similarly, F12 is: F4+F5$\ast SQRT($F9$)-0.00001$.
What are these formulae calculating? F13 simply is: F12$-$F11