Thursday, December 17, 2020

Interdependence stretches the critical path



In project management school, the lesson on Critical Path includes Rule #2:  
"Apply resources first to the critical path and subordinate demands of other paths to ensure the critical path is never starved.
Most of the time, Rule #2 is good advice.(See Rule #1 below) 

Rule #1, if you haven't guessed is:
Create a schedule network so that the critical path is revealed.

If you're only working with major milestones, there is no Rule #1.
It follows that there can be no Rule #2, and so no insight to schedule starvation. Pity.
 
A longer path?
Some of the time, Rule #2 has unintended consequences, like making the critical path longer! How does this happen?

The problem arises when we move from the abstract of 'headcount' to the real world of 'Mary' and 'John'. Alas! The "parts" are not interchangeable. Mary and John are unique. Consideration must be given not only to the generic staffing profile for a task but also to the actual capabilities of real people.

Staffing and Schedule intersection
The intersection of the staffing plan with the schedule plan sometime brings up results that are not always as we want them. Intersection means overlap, and overlap means that the planning elements must be moved about so that each overlap is harmonious.

Take a look at the following figure for Rule #2: There are two tasks that are planned in parallel. If not for the resource requirements, these tasks would be independent, and if independent the critical path would be 50 days -- the length of task 1. Task 2, as you can see, is only 20 days duration.


You can probably see that if not for the specific assignments of Mary and John, the critical path could be as short as 50 days, not 65 as shown.

Let's violate Rule #2 and invent Rule #3: Reorganize the network logic to take into account unique staffing applied to schedule tasks.

 
Using Rule #3, staffing does not actually start on what was the critical path, a violation of Rule #2. 
 
But the advantage of Rule #3 is that the overall schedule is shorter nonetheless. In this case, the critical path is only 55 days.
 
There is still inter-dependence among tasks. But a new critical path using Rule #3 more optimally incorporates the sequencing constraints of the original path and the staffing constraints brought about by Mary and John.


Here's the main idea to take away: 
Any lack of independence among tasks will stretch the path upon which those tasks are scheduled




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