Critical-Path

Critical-Path

 Critical-Path

After hard working day, I believe I need to share the below with some friends who clearly should consider real world behind the theory...  The notion of one critical-path is a theoretical concept, but rarely the case in real life. Most projects have multiple completion milestones, and the date constraint locking in the latest finish for each milestone launches a separate backward pass, resulting in a separate set of late, dates and resultant total float values.
DEFINITIONAL CRITERIA FOR THE TERM, CRITICAL-PATH
Lets examine the term, critical-path, in the context of its most common usage.  Immediately we appreciate that the intent behind the term is to deem one particular path as being (the most) “critical,” as compared to all other paths in the schedule. In so doing, we come face-to-face with why the overall meaning of the term critical-path is so often and easily misunderstood, and why there is so much confusion and debate in scheduling circles as to what IS the “critical path”?  This confusion is fostered by multiple and differing definitions of the term, even 50 years after its invention.  To fully appreciate the limitation of the comparative context, let us consider three different scenarios.

In each, the schedule contains 100 activities, the critical-path contains 20 activities, the project’s contractual length is 365 calendar days, and the project is working a seven-day calendar.  In scenario 1, the longest/least-float path is 386 days long, with total float of -21 (negative 21). The second longest/least-float path is 382 days long, bearing total float of -17. The third longest/least-float path is 378 days long, with total float of -13.  In scenario 2, the longest/least-float path is 352 days long, with total float of +13. The second longest/least-float path is 348 days long, bearing total float of +17. The third longest/least-float path is 344 days long, with total float of +21.  In scenario 3, the longest/least-float path is 382 days long, with total float of -17. The second longest/least-float path is 378 days long, bearing total float of -13. The third longest/least-float path is 352 days long, with total float of +13.

Applying either the longest or least-float path definition to the first scenario, we are quick to identify the 386-day path as the critical-path. Yet, we must also agree that the other two paths, both significantly behind schedule, will not be called critical, but merely “near” critical. Specifically, the path with a length of 382 days is not the longest path – and with total float of -17, which is not the least float -- it is therefore not the critical-path. Yet, in scenario 3, a path of the same length and same total float IS labelled critical-path, simply because, there, it is the longest path, or the one with the least float.

Obviously, our definition makes no sense!

We can see this same contradiction, even more starkly, when we compare the 352-day path of scenarios 2 and 3. In scenario 2, the path with positive total float of +13 is critical because it is the longest path and has the least total float. Yet, in scenario 3, a path of the same length and total float is not critical, because it is not the longest or least float path.  A splash of common sense jolts us back to reality. How can a path that is 17 days behind schedule not be considered “critical,” While a path that is 13 days ahead of schedule is considered “critical?”

Surely there is something fundamentally wrong with our definitions of what is to be considered critical! Aside from the comparative approach being flawed, there is another, equally important, observation to be made from the above examination. Whether “least” or “longest,” there is a fallacy in presupposing that there can only be one critical-path per schedule (even if we set aside the occasional condition where two paths are of the same length, or bear the same total float).

Today, most schedules incorporate more than one completion milestones. Yet, both the longest path and least float path definitions continue to speak in terms of project completion as the one and only objective of “the” critical path. As I described in my previous article of paths and path segments, the introduction of interim date constraints invalidates both the longest path and least-float path definitions of the term, critical-path, as long as the definition speaks only in terms of project completion, and ignore the presence of intermediate completion milestones.

Conclusions

Definitional criteria for the term, critical-path:
• Whatever the basis for determining the critical-path, it should not be comparative. That one path is longer or shorter than another or whether it has total float greater or lesser than another does not help us understand what is truly most critical with respect to the timely achievement of one or more required completion milestones.
• The notion of one critical-path is a theoretical concept, but rarely the case in real life. Most projects have multiple completion milestones, and the date constraint locking in the latest finish for each milestone launches a separate backward pass, resulting in a separate set of late dates and resultant total float values.
• The longest path basis for determining the critical-path is flawed, because of the common use of internal start-no-earlier-than and finish-no-later-than date constraints. From a practical standpoint, the prolific use of date constraints cannot be mandated out of existence, and is likely to only increase in the future. Actually, we encourage the discrete use of date constraints as a practical and effective way to simulate project management intentions and their likely consequences. And,
• The least-float path basis for determining the critical-path is flawed in that it necessarily ignores all but the path with the “least” float. When multiple paths are behind schedule (bearing negative total float), the least-float method describes all but the path with the least total float as being something other than “critical.” If we reserve the term critical-path for the path with the least total float, then we are left to find other words to describe other paths that might have frightfully negative total float values. One popular term is “near-critical path,” but what is near-critical about a path with a negative total float value of -17?

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