Help! Critical Path formula starts with 1 or 0??

 Have a question asking if CPM formula starts with 1 or 0.  Head First states always start with one but answer to this question says always start with 0?  Can anyone shed some light on this please?

Tina

Excellent, thank you so much!  I looked at previous threads on PMZilla but did not find a definitive answer.  This give me confidence for doing problems on the exam.

Tina