Interdependence stretches the critical path

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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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Critical path: who's in charge?

You've got a job to do; you've sequenced and scheduled it

But, in the middle of your critical path there is another independent project (or task) over which you have no control. In effect, your schedule has a break in its sequence over which you have no influence.

Yikes! 

This is all too common in construction projects where independent "trades" (meaning contractors with different skills, like electrical vs plumbing) are somehow sequenced by some "higher" authority.

So, what do you do?

If you have advance notice of this critical path situation, you should put both cost slack and schedule slack in your project plan, but there may be other things you can do.

 

Cost slack is largely a consequence of your choices of schedule risk management. 

Schedule risk management may have these possibilities:

• Establish a coordination scheme with the interfering project .... nothing like some actual communication to arrive at a solution
• Schedule slack in your schedule that can absorb schedule maladies from the interfering project
• Design a work-around that you can inexpensively implement to bridge over the break in your schedule 
• Actually break up your one project into two projects: one before and one after the interposing project. That way, you've got two independent critical paths: one for the 'before' project and one for the 'after' project

 At the end of the day: communicate; communicate; communicate!

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Start on 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. [Rule #1, if you haven't guessed is: create a schedule network so that the critical path is revealed; Rule #1 is ignored by those that work only with milestones]

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'. Now, the parts are not interchangeable. Now we must consider not the generic staffing profile for a task but actual capabilities of real people.

Now we must consider the intersection of the staffing plan with the schedule plan. When two plans intersect, the results 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:

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

Let's violate Rule #2 and invent Rule #3: Reorganize the network logic

Staffing does not actually start on what was the critical path, but the overall schedule is shorter nonetheless. A new critical path is born which incorporates the sequencing constraints of the original path and the staffing constraints brought about by Mary and John.

Obviously, on any network of non-trivial scale you can not really do this by hand; it requires a schedule optimizer and the optimizer has to be configured to try, at least, violating Rule #2.

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