# Process Sigma calculation

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• #30167

Elsa Rodriguez
Participant

In the process sigma calculatrion formula why is there a 1.5  sigma shift and what does that mean?
Process Sigma = NORMSINV(1-((Total Defects) / (Total Opportunities))) + 1.5
Also there is another formula published: process Sigma = 0.8406 + SQRT(29.37 – 2.221 * (ln(DPMO))).What does each constant mean or where does they come from?

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#78330

Perryman
Participant

Hi Elsa
The addition of 1.5 in your first formula is to convert your sigma value into short term.  The common understanding is that in the long term, you will have shift and drift in your process, which will therefore produce more defects and consequently lower your overall sigma value.  Since your formula uses discrete data to start (which is considered long term already), you have to at the conventional 1,5 to reflect a short term sigma value.  Many organizations express their sigma values in the short term, by convention.