Short term and long term Z

Short-term z bench and long-term z bench

Z bench values are used to describe the capability of your process, also known as the sigma capability of your process.

Short-term Z bench (Z benchST)	Long-term Z bench (Z benchST)
Is the number of short-term standard deviations that fit between the process center and the red line. This line represents the point on the normal curve that would include all the defects, if you could put them on one side. (So the defects outside both upper and lower specifications combined on the right side of the curve).	Is the number of long-term standard deviations that fit between the process center and the red line This line represents the point on the normal curve that would include all the defects, if you could put them on one side. (So the defects outside both upper and lower specifications combined on the right side of the curve).

LSL USL	LSL USL
It is a measure of how the current process would perform if you could hold this short-term variation. It is the best case scenario or the process entitlement.	It is a better measure of reality because we can't usually hold the short-term variation over time. It represents the pain the customer feels.

Using the benchmark Z values, you can determine the total defects and easily translate these metrics into DPMO.

Z benchST and DPMOLT are commonly used together.

Note	ZST upper represents the number of short-term standard deviations that are between the center and the upper specification. Likewise, ZST lower would consider the lower specification.

Z shift is the difference between Z benchST and Z benchLT. The larger the Z shift, the more opportunity you have for improvement if you are able to better control your process and eliminate or reduce the special causes that drive the between subgroup variation.
If you cannot calculate the actual shift, you may specify another known shift. Typically, practitioners assume a 1.5s shift (ZST = ZLT + 1.5).