For many years, Six Sigma remained a closely guarded secret at Motorola. The outside world knew about it, but not how to use it. Even today there is a lot of confusion regarding how to calculate DPMO (Defects per Million Opportunities). The confusion lies in the definition and use of opportunities. Several arbitrary methods exist. One method is to use a formula such as DPMO = C+P+M (connections, parts, non-manufacturing steps) or variations of this. Another method is to look at what the customer would consider a reject and count those as DPMO. Either way, the DPMO should be directly tied to CTQ (critical to Quality characteristics).
Defect opportunities can be very arbitrary. If detailed rules for defining the opportunities are not developed and monitored, the opportunities can become instruments for data manipulation and or completely mis-understood. Opportunities for a given process can vary hundreds of percent depending upon how they are created.
I hope this helps clear up your question.
KN http://www.znet.com/~sdsampe/kimn.htm
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