# Explain more detail to me about different Zlt, Zst and Zbench

Six Sigma – iSixSigma Forums Old Forums General Explain more detail to me about different Zlt, Zst and Zbench

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• #29226

Aries
Participant

Dear all,
Thanks,
Aries

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#74229

BB
Participant

Type in the word “bench” and press Search.
You’ll get several articles explaining the terms you mention.

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#74240

C.Vasantharayalu
Participant

Hi Aries,
Zst, and Zlt indicate Short term and Long term process capability, To understand more clearly while working with six sigma projects, we try to calculate capability index – In this context normally the other sources of variation will be in controlled condition (for e.g. if u are studying the capability of a part produced out of moulding machine, you would have probably ensure that right process control measures are taken, trained personnel are employed, environmental aspects if any are under control, proper maintenance activities are carried out) so Zst calculated under these condition will be high and It is known as Zst. In Long run there is probability of getting so many variations induced into the process hence it based on the studies that Zst will shift by 1.5 (i.e Zst-1.5=Zlt)

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#74262

Mikel
Member

z bench is a BS number that I think of as Harry-speak.
Definition of Harry-speak — a language invented to confuse and intimidate, to give self importance to a spin master who has never really done this stuff.
z bench has no useful purpose.

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#74267

Member

Dear Aries,
You will find plenty of confusing guidance on this issue.To enable you to understand let us define Long Term and Short term
1. Long term variation of a process is variation estimated of a process using sampling conditions such that the process has been able to exibit both type of variation (Special Cause and common cause).Typically the ssampling may run for week to week, Month to month.
2. Short term variation of a process is termed as that where the process has not had an ability to exibit the two types of variation .Typiclly this occurs in controlled condition trials, or sampling over only one day.
One thing you must remember that the time period indicated by me are as examples.
Based on this when we estimate the Z value of the process We term as Zst or Zlt.
Generally in most of the cases of the data we collect is assumed to be long term and then based on the Z value obtained we add1.5 to obtain Zst.
Normally the process baseline in Six Sigma is expressed as Zst and the target is 6.
For Z bench we calculate the Z value for USL and LSL of the process.Add the PPM defectes beyond the tails of the distribution and then enter the Z value table using this value to obtain a Z value.This is called a Z bench.
I hope I have clarified your doubts.In case I have not then I have managed to confuse you at an even higher level, well that is what Mikel did