Definition of Z Shift:
Z shift is the difference between ZST and ZLT. The larger the Z shift, the more you are able to improve the control of the special factors identified in the subgroups.
Z shift is usually assumed to be 1.5 (ZST = ZLT+1.5). However it can be computed precisely for any given process by calculating its “Between sub-group variation” using Process Capability Analysis.