# Tag Archives: PDM

## Critical confusion – when activities on the critical path don’t compute……

The definition of a schedule ‘critical path’ varies (see Defining the Critical Path), but the essence of all of the valid definitions is the ‘critical path’ determines the minimum time needed to complete the project and either by implication or overtly the definitions state that delaying an activity on the critical path will cause a delay to the completion of the project and accelerating an activity will (subject to float on other paths[1]) accelerate the completion of the project.

A series of blog posts by Miklos Hajdu, Research Fellow at Budapest University of Technology and Economics, published earlier this year highlights the error in this assumption and significantly enhances the basic information contained in my materials on ‘Links, Lags and Ladders’ and our current PMI-SP course notes.  The purpose of this post is to consolidate all these concepts into a single publication.

The best definition of a critical path is Critical Path: sequence of activities that determine the earliest possible completion date for the project or phase[2].  This definition is always correct.  Furthermore, in simple Precedence networks (PDM) that only use Finish-to-Start links, and traditional Activity-on-Arrow (ADM) networks the general assumption that increasing the duration of an activity on the critical path delays the completion of the schedule and reducing the duration of an activity on the critical path accelerates the completion of the schedule holds true.  The problems occur in PDM schedules using more sophisticated link types.  Miklos has defined five constructs using standard PDM links in which the normal assumption outlined above fails. These constructs, starting with the ‘normal critical’ that behaves as expected are shown diagrammatically below[3].

Normal Critical

The overall project duration responds as expected to a change in the activity duration.

A one day reduction of the duration of an activity on the critical path will shorten the project duration by one day, a one day increase will lengthen the project duration by one day.

Reverse Critical

The change in the overall project duration is the opposite of any change in the activity duration.

A one day reduction of the duration of Activity B will lengthen the project duration by one day, a one day increase will reduce the project duration by one day.

Neutral Critical

Either a day decrease or a day increase leaves the project duration unaffected. There are two variants, SS and FF:

In both cases it does not matter what change you make 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.

Bi-critical Activities

Any change in the duration of Activity B will cause the project duration to increase.

A one day reduction of the duration of Activity B will lengthen the project duration by one day, a one day increase will 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.

Increasing Normal Decreasing Neutral

An increase in Activity B will delay completion, but a reduction has no effect! There are two variations on this type of construct.

A one day increase in the duration of Activity B will increase the project duration by one day, however, reducing the length of Activity B has no effect on the project’s duration.

Increasing Neutral Decreasing Reverse

An increase in Activity B has no effect, but a reduction will delay completion! Again, there are two variations on this type of construct.

A one day increase in the duration of Activity B has no effect on the project’s duration, however, reducing the length of Activity B by one day will increase the project duration by one day.

## Why does this matter?

The concept of the schedule model accurately reflecting the work of the project to support decision making during the course of the work and for the forensic assessment of claims after the project has completed, is central to the concepts of modern project management.  Apart from the ‘normal critical’ construct, all of the other constructs outlined above will produce wrong information or allow a claim to be dismissed based on the nuances of the model rather than the real effect.

Using most contemporary tools, all the planner can do is be aware of the issues and avoid creating the constructs that cause issues.  Medium term, there is a need to revisit the whole function of overlapping activities in a PDM network to allow overlapping and progressive feed to function efficiently.  This problem was solved in some of the old ADM scheduling tools, ICL VME PERT had a sophisticated ‘ladder’ construct[4].  Similar capabilities are available in some modern scheduling tools that have the capability to model a ‘Continuous precedence relationship[5]’ or implement RD-CPM[6].

[1] For more on the effect of ‘float’ see: http://www.mosaicprojects.com.au/PDF/Schedule_Float.pdf

[2] From ISO 21500 Guide to Project Management,

[3] The calculations for these constructs are on Miklos’s blog at: https://www.linkedin.com/in/miklos-hajdu-a1418862

[5] For more on continuous relationships see:  http://www.sciencedirect.com/science/article/pii/S1877705815031811

[6] For more on RD-CPM see: http://www.mosaicprojects.com.au/WhitePapers/WP1035_RD-CPM.pdf

## Scheduling Acronyms – use the correct terms!

Critical path scheduling has only been around for 60 years, is well documented by the originators of the discipline and central to the practice of project management. However, through ignorance, overt commercialism or laziness, far too many scheduling professionals continue to confuse the terms and degrade the practice.

If we can’t use the same correct term consistently for a function or process in scheduling why should anyone else take us seriously. The originators of the various concepts knew what they called each of the items discussed below, it is both professional and polite to respect their intentions and legacy.

CPM = Critical Path Method. (Also called CPA – Critical Path Analysis) This term emerged in the 1960s to describe the two variants of CPM, ADM and PDM. CPM uses a single deterministic duration estimate for each activity (or task) to calculate the schedule duration, activity start and finish dates, various floats and the ‘critical path’. CPM focuses on the activities.

ADM = Arrow Diagramming Method. Also called AOA (Activity on Arrow).  ADM was the first of the CPM techniques developed by Kelley and Walker in 1957.  This style of network diagramming has largely faded from use.

Activity-on-Arrow Diagram

PDM = Precedence Diagramming Method. Also called AON (Activity on Node).  PDM was the second of the CPM techniques developed by Dr. John Fondahl, and published in 1961.  PDM is the standard form of CPM networking used today.

Precedence diagram

PERT = Programme Evaluation Review Technique. PERT was developed by the US Navy in 1957 in parallel with CPM and used an identical ADM network.   PERT differentiates from CPM in several ways.  Its focus in on the probability of achieving an event (eg, the completion of a phase or activity), the expected duration of each activity is calculated from three time estimates using a ‘modified Beta distribution’ (optimistic, most likely and pessimistic).  The ‘PERT Critical Path’ is calculated using the ‘expected’ durations and very simplistic probability assessments can be made based on the variability in the three estimates (but only on a single path).  PERT calculations for the ‘expected’ durations can be applied to PDM networks but are only of value if all of the links are ‘Finish-to-Start’. PERT is simplistic and significantly less accurate than the modern Monte Carlo analysis. For more on this see: Understanding PERT.

Summary

Calling any deterministic CPM schedule a PERT Chart is simply wrong; PERT is defined by three time estimates! Using PERT when you could use Monte Carlo is stupid – the information generated is less accurate. And inventing new names for existing processes is confusing and damaging.