CPM Scheduling – the logical way to error #1

Section 3.5 of Easy CPM looks at some of the logical scheduling errors that are easy to introduce into a schedule, and that for the most part will not show up in the automated checking tools applying test such as the DCMA 14 point assessment (see more on the DCMA assessment at: https://mosaicprojects.com.au/WhitePapers/WP1088_DCMA-14-Point.pdf)

The naming convention used below is borrowed from Miklos Hajdu.  In all cases the links shown in the diagram are the controlling links, in a ‘live’ schedule there are likely to be many other links as well.

Reverse Critical

In this logical configuration, the change in the overall project duration is the opposite of any change in the activity duration.

A reduction of 1-day in the duration of activity B will lengthen the project duration by one day, an increase of 1-day will reduce the project duration by one day.

Neutral Critical Open ends (dangles) have the effect of isolating the activity duration from the schedule. The project duration is unaffected by either a 1-day decrease, or a 1-day increase in the duration of activity B. There are two variants, SS and FF:

In both cases it does not matter what change is made to activity B, there is no change in the overall duration of the project.  This is one of the primary reasons almost every scheduling standard requires a link from a predecessor into the start of every activity and a link from the end of the activity to a successor, however, even with other links in place, if the control is through either of the scenarios above, the result is still the same.

Bi-critical Activities

Finally, for this post, any change in the duration of activity B will cause the project duration to increase.

A 1-day reduction of the duration of activity B will lengthen the project duration by one day, and an increase of 1-day will also lengthen the project duration by one day.  Bi-critical activities depend on having a balanced ladder where all of the links and activities are critical in the baseline schedule. Increasing the duration of B pushes the completion of C through the FF link. Reducing the duration of B pulls the SS link back to a later time and therefore delays the start of C.  The same effect will occur if the ladder is unbalanced or there is some float across the whole ladder, it is just not as obvious and may not flow through to a delay depending on the float values and the extent of the change.

Easy CPM

There are more examples of similar logical inconsistencies included in Section 3.5 of Easy CPM. Easy CPM is designed for schedulers that know how to operate the tools efficiently, and are looking to lift their skills to the next level. The book is available for preview, purchase (price $35), and immediate download, from: https://mosaicprojects.com.au/shop-easy-cpm.php  

6 responses to “CPM Scheduling – the logical way to error #1

  1. Pat, what are you going to be doing exactly a year from today? And what are the odds that you will predict it accurately?

    The N.O.A.A. predicts the path of hurricanes & typhoons. They have the most sophisticated Cray supercomputers running Systems Dynamics models based on hundreds of years of data and the BEST they can predict is 5 days into the future and the “Cone of Uncertainty” is so wide as to be almost meaningless. (See Figure 7- https://build-project-management-competency.com/1-4-1-6-unit-6/)

    In 1895, Field Marshall Helmuth von Moltke told us “no plan survives 1st contact with the enemy” and later on, General Dwight Eisenhower reaffirmed this concept when told us in 1951 that “plans are useless, but planning is essential”.

    As CONTRACTORS, we also embrace these words of wisdom from General Omar Bradley- “AMATEURS study STRATEGY while PROFESSIONALS study LOGISTICS. https://pmworldlibrary.net/wp-content/uploads/2021/11/pmwj111-Nov2021-Giammalvo-principle-based-project-management-the-whole-truth.pdf

    Speaking as a CONTRACTOR where our own money is on the line if our projects “succeed” or “fail” we take your pretty CPM wallpaper and largely IGNORE THEM.

    Why? Because in most cases, the schedule was prepared by “computer jockeys” with little or no field experience, And few if any “planner/schedulers” bother to involve the field to find out what we plan to do (and WHY), how long it will take and what risks/constraints we face. For proof to support my statements of what the field look like and why CPM schedulers are largely useless, just watch any of the gold mining shows on the Discovery Channel-.https://www.discovery.com/shows/gold-rush

    We see your CPM schedules as little more than contractual obligations rather than useful management tools.

    So what do we CONTRACTORS use in real life? We use Rolling Wave Planning (see Figures 17 and 18- https://build-project-management-competency.com/1-4-1-9-unit-9/) (what our IT colleagues call “sprints” or “scrums”) and the “Sticky Note Method” (see figure 6- https://build-project-management-competency.com/1-4-1-9-unit-9/) to plan our deployment of scarce/limited resources where they can generate the most REVENUE in the shortest period of time. (Contractors live and die by our cash flows.

    Based on your own 2020 KPMG/AIPM/IPMA research from Australia https://home.kpmg/au/en/home/insights/2020/08/australian-project-delivery-performance-survey-2020.html indicates that:
    -52 percent of projects are delivered with stakeholder satisfaction
    -51 percent of projects are likely to meet the original goal and business intent
    -48 percent of respondents feel their organization manages projects and programs effectively or very effectively
    -42 percent of projects are likely to be delivered on time
    -40 percent of projects are likely to be delivered on budget.

    So how much longer are we going to “do the same things over and over again yet expect different results?” (Einstein’s definition of insanity)

    • If you actually bothered to read the post (before waffling on about other issues), the inconsistencies occur when a duration is changed. The schedule in question may extend ro 1 hour, 1 day, 1 week 1 month or 100 years into the future – the analytical inconsistencies are still the same.

  2. Pat, the whole premise of CPM Scheduling is a fatally flawed construct and one that needs to be replaced.

    It may have worked for the US Navy to schedule ships but I have yet to see it work on any projects of significant size or complexity.

    There is no way that a linear model can come close to modeling REALITY no matter how many theoretical scenarios you create. Without those pesky feedback loops, the whole thing is a FARCE that is causing more DAMAGE to the practice of project management than good.

    And your own Curtain University’s Peter E. D. Love is an expert on this subject. Why not LEARN from his work?

  3. drpdg, CPM scheduling does not take into account many real life constraints but there are methods and tools that create feasible schedules that can significally improve project plans developed manually. Our experience of using PM software for project management showed very good results.
    The secret is simple: input into the model all existing constraints and uncertainties that people take into account when scheduling and get the result that will be accepted by project management team.
    People will not rely on the results that are produced ignoring real life limitations.
    We have different experience and please don’t state that your own view is the only one that is true.

  4. Pingback: CPM Anomalies Invalidate Monte Carlo | Mosaicproject's Blog

  5. Great book review Pat, and really enjoying the comments. CPM gurus have been discussing the flaws in the process and software for years. An article that started it for us (PMA Consultants) was the ENR Critics Can’t Find the Logic in Many of Today’s CPM Schedules in 2003 (https://www.enr.com/articles/28965-critics-can-t-find-the-logic-in-many-of-today-s-cpm-schedules). Sprints or look-ahead planning has excellent value for contractors, and including schedule risk analysis to capture the constraints is not done enough or done well enough. Sharing a recent presentation from PMA’s Dr. Gui Ponce de Leon on Highly Predictable Schedules (at https://pmatechnologies.com/keynote-highly-predictable-schedules/).

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