TOPIC: Activity Duration Equation clarification

Hi ,

Equations of Network Diagram in specific:

- Activity Duration = EF-ES+1
- Activity Duration = LF-LS+1
- ES = EF of predecessor +1
- LF = LS of successor -1

Ex. Page 90 of PMP Certification Mathematics by Vidya And Riva

9 2 11 15 1 16
D F
19 10 21 21 6 22

ES Dur. EF
Task name
LS Slack LF

Why other books like Headfirst explains such concept while others above cannot see it applying. Am I missing something am talking about +1, -1 in the equations above just want to understand this but it seems different diagrams some support such other not why ?


Thank You.


saad