Generic requirements regarding capability study in automotive


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If my drawing and/or quality manual of my customer says nothing about requirements for capability study, then i must apply the generic automotive requirements.
In this case I know that for short term study, I will apply AIAG requirements.
According to PPAP manual 4th edition the acceptance criteria for initial study (short term) is index > 1.67 (chapter

I know that acceptance criteria for long term (serial production) is index > 1.33 ... but ...

My question is: in what document is specified this value for long term study?