function to calculate a standardized distance to the Upper specification limit (sigma units)
UD(LSL, USL, QC.Center, QC.Sigma)
number, customer's lower specification limit.
number, customer's upper specification limit.
number, the mean or median value determined from an XmR plot or a Studentized (e.g., xBar) analysis.
number, the sigma value determined from an XmR plot or a Studentized (e.g., xBar) analysis.
numeric, standardized distance to the upper specification limit (sigma units)