Critical Chain Project Management – A brief overview

                              The below article will also be available on the Simplilearn Website

Critical Chain Project Management was developed and publicized by Dr. Eliyahu M. Goldratt in 1997. Followers of this methodology of Project Management claim it to be an alternative to the established standard of Project Management as advocated by PMBOK® and other Standards of Project Management.  This article attempts to provide a brief overview of the Principals of Critical Chain Project Management and its applicability to manage Projects across all organizations and verticals.

                The Critical Chain Method has its roots in another one of Dr. Goldratt’s inventions viz The Theory of Constraints (TOC). This Project Management Method comes into force after the initial Project Schedule is prepared, which includes establishment of the task dependencies.  The evolved Critical path is reworked based on the Critical Chain Method. To do so, the methodology suggests and assumes constraints related to each task. A few of them are elaborated as under.

Ø  There is a certain amount of uncertainty in each task

Ø  The task Durations are overestimated by the Team Members or Task Owners. This is typically done to add a safety margin to the task so as to be certain of its completion in the decided duration.

Ø  In most cases, the tasks should not take the time estimated, which includes the safety margin, and should be completed earlier.

Ø  If the Safety Margin assumed is not needed, it is actually wasted. If the task completes earlier, it may not necessarily mean that the successor task can start earlier as the resources required for the successor task are not available until their schedule time. Hence the saved time cannot be passed on to finish the Project early. On the other hand, if there are delays over and above the estimated schedules, these delays will most definitely get passed on, and in most of the cases, will exponentially increase the Project Schedule.

With the above assumptions, the Critical Path Methodology of Project Management recommends pooling of the task buffers and adding them at the end of the Critical path.

              

            The Critical Path Project Management defines 3 types of Buffers

1.       Project Buffer -         The total pooled buffer shown above(Fig 1.1) is referred to as the Project Buffer

2.       Feeding Buffer -       In a Project Network there are path/s which feed into the Critical path. The pooled buffer on each such path represents the Feeding Buffer to the Critical Path(Fig1.2) resulting in providing some slack to the critical Path.

3.       Resource Buffer-     This is a virtual task inserted just prior to critical chain tasks that require critical resources. This acts as a trigger point for the resource indicating when the critical path is about to start.

              

           As the Progress of the Project is reported the Critical Chain is recalculated. In fact, monitoring and controlling of the Project primarily focusses on utilization of the Buffers. Hence the Critical Chain Method, takes in the basic Critical Path based Project Network and Schedule and derives a completely new Schedule.

The Critical Path Project Management Methodology proves to be very effective in organizations, which do not have evolved Project Management Practices. Also, it is seen that the methodology does not advocate multi-tasking and hence in Projects with complex Schedule Networks, the results of implementing the Critical path Methodology have proven to be deterrent to the overall Project Schedule. Additionally, there is no standard method which has evolved for calculating and optimizing the Project Buffers. The Critical Path Project Management Methodology has had a fair amount of success in Manufacturing domains though it has not achieved any noteworthy success in IT Sector.

                Similar in lines with the principals of Critical Chain Methodology, the Event Chain Methodology of Project Management focusses on determining the uncertain events and the chain Reactions they propagate. It is a method of modelling uncertainties and is based on Monte Carlo Analysis, Bayesian Believe Network and other established simulation methodologies. Events when occurred can cause other events triggering an Even Chain, which will effectively alter the course of the Project. Events and Event Chains are identified and a Quantitative Analysis is performed to determine the extent of the uncertainty and the probable impact of the same on the Project. From this exercise, Critical Event Chains are evolved which have the potential to cause the most impact on the Project. Event Chain diagrams are visual representation of the Event and Event Chains and their impact.

It is clear that the neither the Critical Path Project Management Methodology nor the Event Chain Methodology can be considered as alternatives to the standard Methodology for project Management as advocated by PMBOK®. While the Critical Path Project Management Methodology can be at best used as a tool for deriving Project Schedule networks, the Event Chain Methodology for Project Management can be used as a tool for Quantitative Risk Analysis.

Yogeeta Deshmukh   BE, ITIL, PMP 

Scheduling Projects - Critical Path Method

The below article will also be available on the Simplilearn Website

One of the most critical aspects in Project Management is for the Project Managers to arrive at a Project Schedule.  This Project Schedule has to be based on highly accurate estimates and often requires a visual Network Diagram to depict the Schedules. Of the various tools and techniques available to Develop the Project Schedule, Critical Path Method remains the favourite across Project Managers.

The Critical Path Method was developed in the early 1950s by DuPont.  This simplistic design was initially used by various industries for scheduling their Plant and Machinery Maintenance Projects.  Over the years the Critical path Method has been adapted by Project Managers to arrive at their Project Schedules in almost all verticals. The popularity of the Critical Path Method stems from its design which allows Project Managers to

a.       Estimate the minimum Project Duration

b.      Identify critical activities in the Project; where any delay will cause a Delay for the Project.

c.       Calculate Early Start and Early Finish Dates for each activity

d.      Calculate the late Start and Late Finish Date for each activity without compromising on the overall Schedule

e.      Calculate the amount of scheduling flexibility on the logical network path of the Project Schedule

f.        Provide a graphical view of the Project.

Analysis of the Critical Path Method, thus allows the Project Manager to focus on critical path activities. Eg. If resources are to be added to a Project with a view to shorten the Schedule, it makes more sense to add the resources on the critical activities rather than the non-critical activities.

Elaborate descriptions of the various terms used in the Critical Path Method are as under

Critical Path        -              The Critical Path is that path in the Network Schedule Diagram, where the total duration of all the activities lying in the path is longer than that of any other path of the network.    

Critical Activity   -              All activities lying on the Critical Path are Critical Activities.

Float or Slack      -              Float on an activity is the amount of time it can slip without causing any delay in the overall Project Schedule. Hence, by definition, float for any activity on the critical path will be zero.

Early Time          -              This is the earliest time the activity can start.

Early Finish        -              This is the earliest time the activity can finish.

Late Start            -              This is the latest time the activity can start without affecting the Schedule

Late Finish          -              This is the latest time the activity can finish without affecting the Schedule

The steps for calculating the Critical Path are as under

1.       Put the activity diagram in place

a.       Organize a table of activities, their dependencies and their durations as depicted in the figure below.

b.      Translate the inputs from the table created into a Network Diagram. A network diagram derived from the above table is illustrated below.

c.       Calculate the durations for each path. 

Path1.   Start – A – D – E – F – Finish        =             10

Path2.   Start – A – B – C – Finish              =             18

Path3.   Start – A – B – J – Finish                =            13

Path4.   Start – G – H – I – J – Finish          =             11

2.       Locate the critical path

By definition of the critical Path, the Path with the highest sum total duration is the Critical Path. In the above illustration, Path2 viz Start – A – B – C – Finish becomes the Critical Path and all the activities A, B and C become critical activities.

3.       Estimate the Early Start and Finish days

For calculating Early Start (ES) and Finish (EF) days in the critical Path method, we go through a Forward Pass of the Schedule Network Diagram.  The Early Start and Finish Days for the above example are depicted below.

The ES for the first Activity in the Path is always 1.

The EF for any activity is calculated as EF = ES + duration of the activity -1.

The ES for all other activities (not the first activity) is calculated as ES = EF of predecessor activity +1

For activities which have more than 1 predecessor, the greatest EF among the predecessors is used for calculating ES. Refer Activity J in Fig 3.1. The ES for J = EF of B (10) + 1 = 11.

4.       Estimate the Late Start and Finish days

For calculating Late Start (LS) and Late Finish (LF) days in the Critical Path Method, we go through a backward Pass of the Schedule Network Diagram. The Late Start and Finish Days for the same example are depicted below.

                       

We start at the end of the path. All activities at the end will have LF =18, which is the duration of the Schedule (Refer Step 1, Critical Path Duration is 18). Hence activities F, C and J have LF = 18.

The LS for the activities = LF-duration+1.

The LF for all other activities (not the last activity) = LS of the successor activity -1.

For activities having more than 1 successor, the least LF among the successors is used to calculate LF. Refer activity B in fig 4.1.  The LF for B = LS of C -1 = 10.

5.       Calculate float on each activity.

Float is the flexibility you have in the Project schedule. All activities in the critical path have a float of 0. Hence, for activities A, B and C the float is 0.

We then proceed to the next longest path in the Network. In the illustrated example as calculate in Step1, the Path3 is the next longest path.

Float for activities is = Critical Path duration – Current path duration. In this case it is 18-13 = 5. Hence for activities on this path (except for A and B where Float is already calculated) i.e  J float is 5

We next proceed to the next longest path i.e Path 4. In this case Float = 18-11 = 7. Hence activities G, H and I have a float of 7.

We then proceed on the last path i.e Path 1. In this case Float = 18-10 = 8. Hence activities D,E and F have a Float of 8.

Float can also be calculated as LS – ES OR LF – EF.

Another of the advantages of the Critical Path Method is that algorithms can easily be devised to derive Critical Path using Computer Applications.  Hence deriving the Critical Path is no more an elaborate manual effort but most of the Project Management Information Systems like MS Project etc, provide Project Managers with a facility of calculating Critical Paths. 

As is evident from the above, the Critical Path Method for working out Project Schedules will give best results when estimated activity durations have minimal variations. In situations where the activity durations are highly uncertain, within complex Schedule Network Models, the Critical Path Method may come up with incorrect Schedules.

Yogeeta Deshmukh   BE, ITIL, PMP