Z-Scorenoun

The Z-score is the number of standard deviations of demand variability you choose to cover. It is the bridge between a service-level target and an actual stock quantity: choose 95% service and Z is about 1.65.

Because the normal curve flattens at the top, the last few points of service are the expensive ones. Moving from 95% to 99% lifts Z from 1.65 to 2.33 — roughly 40% more safety stock for four points of service.