Effects of Constraints to Critical Path

041816-Critical Path13

“The final process of risk-based management is monitoring and controlling of risks. These two come together all the time. Monitoring without control is exercise in futility. When executing an approved risk-based plan, a good risk-based management practitioner wants it implemented correctly and effectively using optimum resources. Track all risks regularly in order to appreciate current developments. Observe changes in priority, relevance, constraints, assumptions, and at times, objectives. Maintain a good recording system but avoid getting lost in the data (Frago, R., 2015.Risk-based Management in the World of Threats and Opportunities\Chapter 2 Risk-based Management).

Schedule management is a form of risk management.

Identifying and validating project constraints affecting the schedule is an important part of monitoring and control. Constraints for the most part are project assumptions. Treat them with caution and respect.

Once started, the project is actually operating under three big traditional major constraints* of cost, time, and scope (*note that PMBOK had recently came up with six constraints). Constraints imposed on activities are multiple limitations that will most likely further make it challenging and difficult to manage time.


The critical path ultimately dictates the duration of the project. For this reason, a planning and scheduling person has to understand how constraints affect the schedule.

Effects of Constraints on the Longest Path

To test the effect of constraints, we applied Mandatory Finish of 05-Feb-10 to activity C1080 and Finish on or Before 31-Mar-10 to C1160 (Figure 1).

Figure 1 – Constraints Applied to Project Schedule XYZ-1

Despite the application of Mandatory Finish of 05-Feb-10 for C1080 and Finish on or Before 31-Mar-10 for C1160, while the TF values change, the Longest Path remained the same. In this case, the constraints did not affect the original Longest Path.

Figure 2 – Despite applying the constraints, LP remained the same

To make sure whether the above finding holds true, we apply Finish On constraint of 29-Oct-2010 on a singular downstream activity PMR7040 Commissioning Steam Path. This forces the activity to a later finish date. Upon calculation, strings of activities previously belonging to the Longest Path suddenly ceased to be (Figure 3), turning all the bars green.

Figure 3 – Finish On Constraints Removed Criticality from Activities

The application of the Finish On constraint changes the value of the TF, the Float Path, and the Criticality of the affected activities. It is clear now that the Longest Path has changed, affected by the introduction of constraints. This knowledge should be a good starting point for scheduling beginners.

Filtering the result such that only the Primary Critical Path is showing, we now look at just three activities. The original path was finally broken.

This is why the application of constraints must go through the right rationale and not used haphazardly. It has to be a sensible part of the project execution plan and better, the schedule risk model (Figure 4).

Figure 4 – Only Three Primary Critical Activities Remained

Using the same XYZ-1 project schedule with the same constraints (Figure 1), we calculate using TF = Zero.

 As expected, a broken critical path in the network typical of a schedule with constraints appeared. The network path is continuous; i.e. without break from October 2009 to September 2010. We observe that the last activity to which previous schedule calculation referred to has disappeared from the path.

Critical path was broken after C1190 Install piping assembly. It is the direct result of constraints applied to just two activities. Perhaps, the full appreciation of what constraints do to CP has now started to dawn on our readers. Instead of starting from the data date of October 2009 and completing on September 2010, this path has abruptly ended on July 1, 2010 (Figure 5). The effects were undeniable.

Figure 5 – Abrupt end to CP due to Constraints

When this sample schedule has all constraints removed from the project schedule, the critical path (CP) criteria of TF = Zero changes complexion and follows closely what the Longest Path shows. It might not be the same exactly and with a few missing activity but the alignment is close enough to effectively managed the project.

For an unconstrained and already-started schedule, the critical path should show a list of cascading activities from the data date down to the last activity. The path does not have break between the data date and the last deliverable.

Can you imagine what will happen when the numbers of constraints are increased? What it would do to your schedule? Will your schedule still represent the most likely? You are right! A schedule riddled with constraints can become so problematic it could ultimately lead to project failure.

Changing Schedule Attributes

When a planning and scheduling person changes the attributes of an activity, he changes the schedule attributes and will most likely change the critical path. Updating a schedule is one. For example, it is normal occurrence to update the schedule with actual data periodically. It takes more than just basic planning and scheduling knowhow to make a reliable and intelligent update. Ensure that a qualified person does the job.

Updates to the schedule are changes to the schedule. They include the application of soft constraints which was discussed earlier (Finish on or after, Start on or after, Finish on or before, Start on or Before, and hard constraints (mandatory dates: Finish on and Start on).

Using constraints does not mean it is wrong to do so absolutely. The fact of the matter is, it might sometime be the best and only way to model the current project information or would-be event.

What is important here is to know what, when, where, and how to do it, and to be aware of the possible risk consequences.

Never operate the Primavera tool (or any tool) blindly without understanding the planning and scheduling processes involved. Plan to schedule and schedule to plan (Frago, R., 2015.Plan to Schedule, Schedule to plan).

As Late As Possible

As late as possible (ALAP) is another form of constraint. However, ALAP has a special place in the heart of many planners/schedulers.

In recent years, I noticed that “as late as possible” constraint has become very popular. The use becomes more prevalent when the JIT (Just in time) philosophy was at its zenith. It was reported that JIT was introduced in the west’s manufacturing industry in the 1980 (Wikipedia, 2015.Just-in-time Manufacturing).

There was a resurgence of popularity in the last ten years or so when JIT becomes part company strategies addressing project construction logistics, delivery, material and inventory issues; e.g. module setting JIT, major equipment installation JIT, parts and materials JIT, etc. To JIT, ALAP is friend.

As Late as Possible constraint is a restriction imposed on a scheduling activity or work unit with positive float that allows it to start as late as possible without delaying the successor activities. This constraint sets the early dates as late as possible without affecting successor activities (P6 Project Management e-Help, 2010).

As can be seen on the example below (Figure 6), the application of “as late as possible” constraint forced the critical path (longest path) to shift from GFCBA to GFECBA. Activity E suddenly found itself on the path.

When the project does not give much thought to how it applies the ALAP constraint, the complexion of the project schedule changes. The creation of artificial critical path (not the real critical path) is the end-result.

This becomes especially concerning during blanket application of ALAPs; i.e. ALAPs applied to a bunch of like category activities such as delivery of equipment and their respective installation. It is convenient for the scheduler but does not guarantee effective time management (Frago, R., 2015).

Figure 6 – ALAPs Influences Schedule Critical Path


Frago, R. (2015).Risk-based Management in the World of Threats and Opportunities: A Project Controls Perspective

Frago, R. (2015).Slideshare.com\Understanding ALAP.Slide No. 4

Periodic updates to capture actuals, expected dates (i.e. forecast dates that I often refer to as “forced-cast dates” for obvious reasons), external dates, and other contrivances often create an artificial critical path that calculates against many endpoints.

The Unwanted Result

Pardon the harsh assessment but I dare say that such undisciplined scheduling practices and set-up grow into the root causes of untenable schedule.

Schedule constraints are analogous to big boulders placed over the path of a river. It hinders the natural flow, create whirlpool, eddies, and turbulence causing erosion to land and presenting dangers to those who venture near it.

They introduce inherent risks in the schedule that in many instances force activities to be more critical than they should. We must all remind ourselves to use constraints sparingly and sensibly with backed up rationale.

It has to be supportive of the final intent and not a reflection of foolish whim and empty notion.

Additional Reading

Setting Critical Path by Rufran C. Frago

Schedule Critical Path by Rufran C. Frago


Rufran C. Frago – Author (Rev. 1, 18-Apr-16)

Rufran is the author of the book Risk-based Management in the World of Threats and Opportunities: A Project Controls Perspective.

For those who are interested, please join Rufran at (click hyperlink) the following sites.

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About rcfrago

Rufran C. Frago, P. Eng., PMP, CCP, PMI-RMP, is the Founder/CEO of PM solution Pro, a tradename/trademark of Risk-based Management and Services Inc., a Canadian company focusing on project/business risk-based management consulting, business development, creative products, and training services. Senior project consultant, planner, scheduler, engineer, business developer, published author, with deep, diversified experience in management and controls. Hands-on work experience in engineering, maintenance, reliability program, operation, EH&S, training, and administration. Recognized subject matter expert by peers in project and Turn-around maintenance planning and scheduling, schedule quality assessment, and risk analysis. Proficient in project management framework, processes, tools, systems, and applications. Risk-based practitioner who value safety, integrity, ethics, and professionalism. Mr. Frago's personal work philosophy: "My client's success is also my own!" He has many years of international industry-related work ‎experience in Oil & Gas, Petrochemicals, Oleo-chemicals, Sugar Refining, Power/Utilities/Nuclear, Manufacturing, Consulting, and Education. He has worked in various parts of the world (Location: Asia, Middle East, Canada, and North Africa). Rufran has worked with Caltex, Uniman, Unichem (now Cocochem), ARAMCO-KSA, Central Azucarera de Tarlac, Arabian Gulf Oil Company-Libya, Batangas State University, Saint Bridget's College, JG Summit Petrochemicals, Halliburton-Kellogg, Brown and Root, OPTI Canada, and Suncor Energy Inc. His expertise includes risk-based project management, risk analysis, planning & scheduling, cost management, auditing, maintenance, operation, EH&S and reliability engineering. He is interested in providing solutions and innovations to all clients and stakeholders. Mr. Frago is a risk management professional. He studied at BSU graduating with a Diploma in Petroleum Refinery Maintenance Technician, and BS Mechanical Engineering. He also has a Bachelors degree in Management Engineering from UB. He took up MBA courses under UP-PBMIT Consortium. Rufran completed Computer Technician Program at ICS-Pennsylvania, USA, APM Certificate program at SAIT-Calgary, and PM Certificate program-Construction Management. He is presently taking up PM Certificate program-Risk Management at University of Calgary. He was a recipient of the Gerry Roxas Leadership Award, the American Field Service (AFS) Scholarship grant, and the CALTEX scholarship grant where he specialized in Petroleum Refinery Maintenance. The author wants to share his knowledge and leave behind some legacy to all readers, most especially to his wife, children and lovely grandchildren, Eva and Mia.
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1 Response to Effects of Constraints to Critical Path

  1. Pingback: Setting the Critical Path | Your World, Our Risk Universe

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