I will show you different behavior in MS PROJECT 2010 in Critical Path when you have, and when you do not have Elapsed days in duration fields!
Suppose that you have three Tasks and each of them has one day duration and they are connected:
I will apply Tracking Gantt View to see Critical Path:
Finally, I will get:
You can see that all three Tasks are on Critical Path. Why? Because Critical Path is Path which can have no delays. That means if any task is behind the schedule (late), the whole project will be late. So Task1 one should start at 11.07.12 (from 08:00 till 17:00 with one hour for lunch), Task2 should start at 12.07.12 (from 08:00 till 17:00 with one hour for lunch), and should start at 13.07.12 (from 08:00 till 17:00 with one hour for lunch)!
Now suppose that Task 1 has duration 1ed (1 elapsed day, e.g. 24 hours):
Everything is the same! Task 1 will start at 11.07.12. at 08:00 and will finish at 12.07.12 at 08:00 (24 hours). That looks fine! But what if I have Task2 with duration 1ed instead of Task1?
Task 2 is no more on the Critical Path! How can it be? Is this an error? No, not at all!
I will put some more columns in my Tracking Gantt View: Early Start, Early Finish, Late Start, and Late Finish:
- Early Start: Task 2 has Early Start immediately after Task 1 is finished, and that means it can Start at 11.07.2012, at 17:00 (5:00 PM).
- Early Finish: Task 2 has Early Finish 24 hours (1 eday) after Early Start, and that means 12.07.2012 at 17:00 (5:00 PM)
- Late Finish: Task 2 has Late Finish just before Task 3 must start, and that means it can Start at 13.07.2012, at 08:00 (08:00 AM).
- Late Start: Task 2 has Late Start 24 hours (1 eday), before Late Start, and that means 12.07.2012 at 08:00 (8:00 AM)
The conclusion? Tasks 2 has Slack or Float!
Float is Late Start – Early Start = 12.07.2012 08:00 AM – 11.07.2012 05:00 PM = 15 hours or Late Finish – Early finish = 13.07.2012 08:00 AM – 12.07.2012 05:00 PM = 15 hours
And since Task 2 has a Slack or Float it is NOT ON THE CRITICAL PATH, because on Critical Path Tasks have Float or Slack = 0!