Hi,

We say that Six Sigma corresponds to 3.4 defects per million opportunities, but mathematically speaking, 3.4 DPMO corresponds to 4.5 sigma levels. Why do we do this 1.5 sigma shift ??

why not call it 4.5 sigma.. or what is so special about '6' is six sigma.. ??

pls share your thoughts.

Viswanathan.

Hi Vishwanathan

 

This 1.5 sigma shift corresponds to the difference in Short term Sigma level and Long term Sigma level.

 

Sigma Long Term = Sigma Short Term + 1.5 Sigma

 

It is hypothesized that over a long period of time things could deteriorate due to poor process control or due to special causes that affects process performance. Hence, a process that is operating at six sigma in the short term will only operate at 4.5 Sigma in the long-term. Thus, a process that is operating at six sigma (in the short term) corresponds to 3.4 DPMO in the long term!

Hi Vishwanathan,

 

 

Aayush has provided a good explanation on 6 sigma instead of 4.5 Sigma.

 

 

 

Zst = 1.5 + Zlt

 

 

st=short term lt=long term

 

 

As across subgroup variation is high, the performance over a long period of time Is poorer as compared to short term.

 

 

 

 

For approximate estimation of short term performance, 1.5 is added to the long term Sigma level.