Critical Path Myth, Part 2: Diffusion and Infusion


Blog Series Abstract

The widespread faith in the Critical Path Concept is not justified by the facts. Still, those with a vested interest in their projects finishing timely continue to support the Critical Path Concept and embrace recommended practices that they are told will lead to more successful projects. This three-part blog series is intended to shed light on the Great Delusion under which most Project Management stakeholders unknowingly suffer.

Myself hardly naïve, I fully expect disagreement with what is written in these three blogs. The resistance expected from Critical Path Concept proponents is both understandable and anticipated. I am comforted by a saying in the Air Force: “if you’re drawing flak, you must be over the target.” I remain hopeful that the discussions and debates that this Blog Series is certain to provoke will nonetheless facilitate the kind of open, honest, and informed dialogue needed to at last turn around the ship upon which we all sail together. The truth shall set us free.

About Critical Path Myth, Part 2: Diffusion and Infusion

Despite its many technical limitations and shortcomings as explored in Part 1, those with the most to gain from keeping the Illusion alive are responsible for a Diffusion of standards, best or recommended practices, guidelines, training, and certifications which they know, or should know, to be engulfing an inherently flawed technology. The consequence of their tenacity, persistence, and sweeping influence is a dogged and constant Infusion of the Critical Path Concept into nearly every nook and cranny of the Project Management model.

In Part 2, we will continue our analysis of the current state of Construction Project Time Management by observing how the Critical Path Concept is inextricably woven into the very Project Management fabric.  We will be shocked to find out that — only compounding the technical limitations discussed in Part 1 — the practical application of the Critical Path Concept has merely introduced further deficiencies and convolutions. Said bluntly, how we employ the Critical Path Concept has only made matters worse!

Recap of Part 1

In Part 1, we discovered that the Critical Path Concept, at its most fundamental, suffers one major limitation: it is fraught with uncertainty. And only compounding its intrinsic deficiencies, the Scheduling Community seems gripped in disharmonious, inflexible, and possibly irresolvable debate. Combined — the root technology itself, as well as how we currently manage it — the Critical Path Concept should not rightly inspire the level of confidence and enjoy the reputation for certainty that it currently does. As a quick recap, here are some of the many chilling truths revealed in Part 1. If you have not already done so, I strong encourage you to take the time to read Part 1.

The Critical Path Concept Suffers Widespread Terminological Deficits

Throughout the Project Management community, there is hardly consensus on the meaning of basic terms like Activity, Duration, Path, Critical, Critical Path — or even secondary terms like lead, lag, constraint, restraint, event, deadline, milestone, window, calendar. Even the correct meaning of the word “schedule” is continues to be contested among Scheduling authorities and practitioners, alike!

Disagreement Over Key Computed Critical Path Method Values

Beyond lexicon, there is equally passionate debate among seasoned practitioners and experts as to the correct formulas or derivations for some of the most basic computed Critical Path Method values:

  • Critical Path: There is anything but agreement on just how to recognize a “critical path.” This may seem hard to believe, given that CPM is a 50 year old concept. Yet, today, the arguments rage on over whether it is the Longest Path through a schedule, the Least Total Float path, or even a hybrid concept of sorts … the Driving Path. We learned that all three of these definitions have their proponents and each enjoys fairly widespread acceptance. And yet, they can’t all be right. In fact, as research shows, they mutually invalidate one another!
  • Longest Path: Quite typically the Longest Path has different Total Float values along its length.
  • Least Total Float Path: The Least Total Float Path is not always the longest Path through the Schedule. In fact, quite often it does not even terminate at the end of the Schedule; many times it does not commence at the start of the Schedule!
  • Driving Path: The Driving Path purposely disregards Date Constraints and renders a Path that can be other than the Least Total Float Path or the Longest Path, depending on how you determine Path length.
  • Total Float:  We learned that there are four popular formulas for deriving Total Float, and yet each yields a different answer. Since the formulas are mutually exclusive of one another, they can’t all be right.
  • Early Start: Finally, we learned that there are different ways to compute an Activity’s Earliest Start.  One of those ways actually yields Earliest Dates that are not the earliest that an Activity can start!

Confusion Within the Context of the Critical Path Concept

In the last section of Part 1, we discovered that CPM Schedules are inherently inaccurate. This comes as quite a shock, since the overwhelming impression made by decades of Project Management literature, training, and lectures is that CPM is a very precise methodology. This may be true but, as we learned, precision is not the same as accuracy.

  • Durations: We were reminded that the Activity Durations are estimates. Even the underlying factors that are used to estimate Durations are themselves estimates: e.g., crew composition, productivity rates, and scope of work.
  • Dependencies: It was revealed that the relationships between the Activities — logic ties that weave the Schedule’s Activities into a cohesive network — are themselves reflections of assumptions about how actual Project Execution might play out. We also were told that all of the relationship in a Schedule, in total, only represent a fraction of all of the inter-dependencies that exist in every Project.
  • Conditions: We were reminded that real life conditions cannot be known ahead of time. Thus, our Schedules are only crude assumptions about how future conditions might unfold on the project.
  • Contingencies: Even the time contingencies that we place into our Schedules in an effort to infuse more realism … are themselves estimates.  Moreover, the precise placement of those contingencies only further contribute to the uncertainty of the Schedule.
  • Schedule Maintenance: The process of maintaining the Schedule introduces more assumptions and estimates.
  • Three-Point Duration: Even the more rigorous three-point Activity Duration, as popularized by PERT, is yet another set of assumptions and estimates, albeit complex and impressive ones.

A Poor Choice for Premier Project Management Ideology           

Given everything just said about the Critical Path Concept, it is hard to understand why the Project Management community would select it to be the functional centerpiece of its Project Time Management machine. But there it is, the facts notwithstanding, masquerading as a methodology of great precision and credibility. That it does not deserve the deference it has been shown is only half the message. The other half is that as long as Critical Path remains king of the mountain, no other replacement methodologies will be given serious consideration.

A Short Discussion about Schedule Data Credibility

In my most recent book, CPM Mechanics, I devote an entire chapter to something called the Schedule Data Credibility Profile. This Profile, when fully understood, reveals fascinating insights about the nature of Schedule Data, and which conditions make it more or less credible. In this blog, I won’t get into concepts like Schedule Tense or the five different categories of data to be found in a schedule: Gleaned Data, Realized Data, Apparent Data, Strategic Data, and Predictive Data. Rather, I will stay on point by mentioning a few key axioms explained in the book, truisms that have direct bearing on the subject of this three-blog series.

Schedule Credibility Improves Over Time

This should be fairly obvious. At the very start of a Project, everything in the Schedule is predictive of the future, speculative. But as the Project progresses and the Schedule is “statused,” a greater proportion of the Schedule’s content converts to “actualized data.” With Schedule Data Credibility expressed as a ratio of Actualized Data to Speculative Data, it should be clear that at Project Commencement, Schedule Data Credibility is between 5-10% (assuming that the Schedule is created just before Project Commencement). By the time of Project Completion, Schedule Data Credibility approaches 100%.

Speculative Performance Data Degrades After Ten Weeks

With respect to Schedule content aimed at speculating on what performance will be achieved in the future, predictions for the Imminent Period (roughly 5-10 days past the Data Date) are the most credible.  Beyond ten weeks, predictions are mostly wild speculation.

Schedule Level of Detail Proportional to Schedule Data Shelf Life

ICS-Research has developed a convenient rule of thumb which says that Schedule Data Credibility is achievable if the schedule’s level of detail does not drop below 10% of the length of time across which the data is forecast. For example, for a Schedule covering an 18 month Project, a credible level of detail would be where Activity Durations average no less than 1.8 months, or roughly eight weeks. As Activity Durations are pushed below this threshold, the credibility of such Duration estimates dissolves. (Keep this in mind when we talk about WBS and Cost Engineering requirements, later). The take-away from this paragraph is that too much detail too early in the Project life cycle results in a Schedule with questionable credibility.

Understanding the Real Meaning of Total Float and Critical Path

I brought up the ICS-Research findings about Schedule Data Credibility Profile as a segue to this next, all-important topic. Most Construction Executives and mid-managers (and even many rank and file Project Controls practitioners) do not really appreciate just how impossible a “critical path” is to perform without slippage.  Consider that Total Float measures the gap between two irrelevant temporal extremes. If you read nothing else in these three blogs, re-read the previous sentence! Total Float measures the gap between two irrelevant temporal extremes.  Now, let me defend the statement:

Earliest Dates Represent Extremely Unlikely Performance

Let’s reduce a Project Length to something more understandable.  Let’s scale it down by 100%.  So, an 18-month Project equates to a period of 5.475 days. With 24 hours in a day, that converts to 131 hours. Now let’s assume that the Schedule for that 18-month Project had Activities with an average Duration of ten-days, or 240 hours. If we divide that by 100, as well, then our Reduced Project would have Activities with Durations averaging around 2.4 hours, or 144 minutes.

Now let me ask you to take pencil and paper in hand, and start drawing out a flow chart that will “schedule” everything you will be doing for the next 5½ days. And get down into the nitty-gritty, such that the Activities in your Schedule account for what you will be doing every 2½ hours. How many Activities might your Schedule have, even if there was only one single, continuous path of Activities? Well, in 5.475 days there are 131.4 hours. At 2½ hours per Activity, the Critical Path would be 52 activities long!

Now, here’s the drum roll question: what is the likelihood that every one of those 52 activities will take place precisely as planned? That each will take no more, and no less, than the Duration you estimated? That each and every Activity will take place in the exact order you specified? What is the likelihood that your Schedule is perfect and that it can be, and will be, followed down to the minute? Well, that is what Earliest Dates are all about. Earliest Dates are calculated through an arithmetic process (Forward Pass) which assumes that all prior activities will consume the exact amount of time reflected in their Durations, and that all dependencies between Activities will be honored! No exceptions!

Do you believe that? Prior to this blog, did you realize that this is what Earliest Dates are all about? Folks, they reflect a theoretical best case scenario. How often does that happen in Life? And that is why I call it an extreme. That is also why I call it irrelevant, because no sane Project Manager (or Construction Executive) would ever expect not to run into “surprises.” As we all know, stuff happens!

Latest Dates Represent an Extremely Irresponsible Management Strategy

Latest Dates are derived by a reverse process (Backward Pass) where, working backwards from the Project Completion (or other contractual) milestone, Activity start and finish dates are calculated that represent the last conceivable points in time to which Activities can be delayed without delaying one or more downstream milestone.

Most of us have seen definitions of Latest Dates along these lines. But what is rarely put into print is the underlying assumption behind Latest Dates, which is the same ridiculous notion behind Earliest Dates: that everything will proceed flawlessly, with no hiccups, no problems, no shortages of materials or labor, no missing information, no logistics logjams, no problems whatsoever. Yeah, right!

How many of you would manage your project to Latest Dates? So, once again, I defend my earlier statement. For, just as with the Earliest Dates, the Latest Dates are also extreme, and they are irrelevant, since no responsible manager would ever wait until the last possible moment, with hopes that from that every Activity in the Schedule will be performed flawlessly, and there will not be even one setback.

Zero Total Float Critical Path = Broke Before We Start

Think about that means, folks! For an Activity where Total Float is zero, the earliest that the Activity can be performed is also last possible moment that it can be performed. Is this a good situation? I hope you would agree with me that a Critical Path with zero Total Float is a very undesirable situation. And when this situation occurs naturally in the course of a Project, our proper instinct is to give it our utmost attention. But why, pray tell, would we ever consciously force Earliest Dates to be the same as Latest Dates?

Dear Construction Owner, can you please tell me why your Contracts actually require the Contractor to strip away all positive Total Float along the Critical Path in his Baseline Schedule? (More on this in Part 3.) Isn’t this policy about as rational as requiring a start-up company to commence operations with zero money in the bank? The point of this short digression is to point out that the Critical Path Concept glorifies a zero Total Float Critical Path. In fact, many scheduling authorities actually define the Critical Path as the Path with “zero Total Float.”

Critical Path is a Comparative Value

Next, I want to point out that, no matter how one defines a Critical Path, it is a comparative value. Either it is the longest Path through a Schedule, or it is the Path with the least Total Float. The problem with comparative values is that they are rarely constant. And things turn from bad to worse when we wrap Policy around wavering values. For instance, we tell Project Managers to concentrate on the Activities that reside on the Critical Path. Well, one month Path X is deemed critical (either the longest, or bearing the least float), but  in the next month Path Y is longer or has lower Total Float. Suddenly Path X is no longer “critical;” attention moves away from it. Far too many Project Managers and Site Superintendents know what it is like to be jumping through hoops in an effort to “focus on the Critical Path Activities.”

Damn, the torpedoes! Full Steam Ahead!

Or … “don’t confuse me with the facts”

Despite all of the above red flags, the staunchest proponents of the Critical Path Concept boldly persist in their promotion of CPM books, courses, training, certifications, seminars, standards, best practices, and guidelines. The subtitle to this blog includes two accusations: diffusion and infusion. According to the Online Dictionary, to diffuse means to “spread or scatter widely or thinly; disseminate.” And that is precisely what the Project Management world has been doing for many decades now. They have “spread the word” far and wide; one is hard-pressed to find anyone even remotely involved in Construction Project Management who has not heard about Total Float and Critical Path.

Meanwhile, Off in the Corner … the Arguments Rage On

This seemingly unstoppable marketing of the Critical Path Concept might lead you to believe that it is a fantastic system with no flaws. But the truth is that, on top of all that we have mentioned in Part 1 and above in Part 2, among the Scheduling Elite a whole basketful of other technical issues remain hotly debated, and yet unresolved.

I want to be clear about a few things. I think debate is a good thing. I know that there is more than one way to skin a cat; that there can be more than one good way to do this or that Project Time Management process. So, when highly experienced practitioners get in a room, it is inevitable that there will be disagreements … passionate disagreements. I think that the good work being done by various professional organizations to help consolidate and formalize recommended best practices and guidelines is also commendable effort, and my hat is off to my respected colleagues for all of their hard work. I applaud volunteerism; otherwise, I wouldn’t have given years of my life in voluntary service to some of these international Project Management organizations.

Are We Really Still Debating These Things?

All of that said, I still think it is symptomatic of a discipline in freefall chaos that, after 50 years, the leading experts in Project Time Management should still (in 2013) be debating technical matters that should have been nailed down decades ago. For those of you on the fringes, did you know that we technocrats are still arguing about:

  • What is better to use, Retained Logic or Progress Override
  • What is the best use of Multiple Calendars, or of they should be used at all
  • What is the better expression of progress, Percent Complete or Remaining Duration
  • Whether, how, and where to include weather contingency into a construction schedule
  • The appropriateness of negative Leads and Lags
  • Whether to use the advanced dependency types of Precedence Diagramming Method (PDM), and if so, to what extent?  Should the use of Start-to-Start and Finish-to-Finish be discouraged or restricted?
  • What is the best Total Float measurement: Start Total Float, Finish Total Float, Most Total Float, or Least Total Float?
  • What is the recommended setting for durations: Interruptible or Contiguous?
  • What is the recommended policy on use of Date Constraints? Should they be discouraged, even outlawed?  How many is too many?
  • What is the best advice on the use of Zero Total Float? On Zero Free Float?
  • Who should own the Total Float in a construction schedule?
  • What constitutes a Minor Adjustment to a Schedule (that would not require Owner permission to make), and what constitutes a Major Adjustment (that would require prior Owner permission to make)?

No doubt, continued disagreements over topics like these have been a primary impetus for the efforts of various international bodies to bring consensus and order to a discipline sorely need the same. However, I privately wonder whether the pursuit is an impossible one.  To my mind, it comes down to the breadth and complexity of the subject matter.

All of This In-Fighting Doesn’t Help

On the one hand, I think that a cohesive set of general standards and guidelines, if done right, could benefit Project Time Management. But, on the other hand, I also think that each industry should write its own standards, best practices, and guidelines. This is because Projects across industries and continents are so divergent that one central doctrine intended to cover all industries and geographies would either be too general to be insightful, or too specific to be useful. That said, I still applaud the gallant efforts of so many of my friends and colleagues. They are certainly moving the rock in the right direction.

That said, I cannot close this topic without mentioning the ongoing competition between many of the major international bodies, all jockeying to become the “King of the Mountain.” Rather than working together, these organizations are running a mad footrace to see who can establish and sustain the most successful credentials related to Project Controls. I know of four equally-respectable institutions that, each, feel that their approach to training and certification is vastly superior to the others. Yet underneath fairly superficial differences, they are all saying the same basic things.  Why can’t they work together?

For as they battle on, trying to outdo one another, the immediate victim is the Construction Project itself, which of course translates to the stakeholders of those projects. But the collateral victims are the Project Controls practitioners who don’t know which doctrine to belly up to and which to walk away from. Meanwhile, the institutions are certain to come out with doctrines that will differ from one another in just enough ways (a) to make it difficult for the individual to know which one to embrace, and (b) to continue to fertilize the grounds of confusion and chaos that have plagued Project Time Management for five decades!

On to Part 3

In the third and final blog, we will look at how Construction Executives who have been sipping the Kool-Aid much too long are actually further encumbering their Projects with policies, restrictions, and mandates that, while well-intended, work at cross-purposes to effective Project Time Management. We will conclude Part 3 with suggestions of a few simple things that Construction Executives can do to dramatically improve the temporal outcome of their projects. See you in Part 3!


3 Comments to Critical Path Myth, Part 2: Diffusion and Infusion

  1. Zach Reed says:

    Good reminders of how total float is calculated: forward pass and backward pass. One should remember that the project schedule is based on performance completing exactly as planned. Also interesting points about the inconsistencies and lack of agreement within the industry. One would think that many of these points would be resolved by now.

  2. Sue Backiel says:

    This blog makes some interesting points on deficiencies in regards to terminology, methods and technical issues. It seems to me, all would benefit if these issues were addressed.

  3. Michael Neal says:

    [C] Just to touch on retained logic. As mentioned there is debate as to out of sequence work and retained logic. When an area is put on hold by the owner is it better to proceed to another area thus breaking logic. Stud walls can be framed in part B if part A has a portion on hold, what schedule related should be done if any.

What is your opinion? We'd like to know!