Thursday, July 02, 2015

Primavera P6 - Critical Path is Not Always The Longest Path!



I come across this statement many times – "Critical path is the longest path in the project's network diagram." Yes, true, but partially! Sometimes, a shorter path in the network diagram can be a critical path and the activities on it can be critical activities. 

Let us see how.

I have a project as shown below in Primavera P6. There are 6 activities and 2 milestones with duration, free float, total float and the network diagram shown.

Pic - 1: Critical Path, the Longest Path
The paths in the network diagram, along with their lengths are:
Start  -- Activity A --  Activity B -- Finish  = 0 + 2 + 3 + 0 = 5 days
Start  -- Activity C --  Activity D -- Finish = 0 + 4 + 3 + 0 = 7 days
Start  -- Activity E --  Activity F -- Finish = 0 + 5 + 3 + 0 = 8 days

Obviously, the activities on critical path are E and F, which are highlighted in red and shown in the above Gantt chart. It is the longest path and that is what the typical textbook definition says. Also, all activities on the critical path will have total float of value zero. Is not it?

Now, I applied a primary constraint for Activity B - "Mandatory Start" and set the constraint date to June 30, 2015. Note that for Activity B, the planned start date was July 1, 2015. 
Pic - 2: Apply mandatory start constraint to Activity B

I rescheduled with this constraint and this how the Gantt Chart and its corresponding table looks in the "Classic Schedule Layout" of activities screen in Primavera P6. Here, Activity A and B are also critical path activities!

Pic - 3: Critical Path with Longest as well as Shorter paths

So, what happened?
Activity A is now a critical activity as the total float has now become negative. Earlier, in pic - 1, the float of Activity A was 3, but post constraint set for Activity B, the float of A has changed to "-1". For Activity B, total float has changed from 3 to 0. And both are highlighted in red,i.e., they are critical path activities. (in Pic - 3) 

Is this path "Start  -- Activity A --  Activity B -- Finish" the longest path? No. The complete path length is definitely not the longest one. But it is now a critical path, too! It is because total float of A has turned negative.

So, here are the things to note for critical path:
1. Critical path can be the longest path of the project, which determines the shortest duration of the project. Here the total float of the critical activities will be zero.
2. Critical path can also include activities whose total floats are less than zero. In this case, the project duration may not be the shortest or the path may not be the longest one.

The second part, as mentioned above, is mostly missed by management practitioners. The important point to remember is that if the total float falls below zero for an activity, then also, the activity can be in a critical path. In others words, critical path can have activities with zero or negative total float. So, when you have negative total float, be careful.

It does not matter which project management software you use. These are the concepts on critical path. It can have zero or negative float. Also to note, if you are preparing for PMP exam, be clear about these concepts. 

Tip:
To choose, what type of critical path you want in Primavera P6, change the settings of the project. Go to Project screen -- bottom Details and check the Settings tab. 


If you want to have the longest path as critical path and do not want to consider total float, then you can use this setting. However, the total float option is advisable. Why? It is because during risk analysis, the activities having negative total float will show a high "criticality index" as compared to the ones with zero total float. This we will see in the next post.


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7 comments:

  1. Why the duration of ACT D is 3d but it is finished on 07-Jul but not 05-Jul?

    ReplyDelete
    Replies
    1. According to the free float formula, the free float in your example is equal to -1.

      How you can expalin the free float is equal to 0.

      Delete
    2. According to the free float formula, the free float in your example is equal to -1.

      How you can expalin the free float is equal to 0.

      Delete
    3. Free float is not -1. Total float is -1.
      Total float can be 0 or -1 for critical path activities. Which activity in particular are you referring to?

      Delete
  2. I am referring for activity A. In your example the free float is equal to 0. According to the free float formula, free float for activity A is equal to -1.

    How can you explain this free float is equal to 0 ?

    Thanks

    ReplyDelete
    Replies
    1. I've changed the constraint. And I am calculating the critical path primarily considering total float. In this case the above shown values will come. There are options in Primavera to check how you want to calculate multiple critical paths - with total float or free float (if you want with free float, the way of calculation changes). I'm using the default one - total float (<=0h), and have total float calculation options - default ones. You can change the options based on your need.

      Delete

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