An optimized schedule is good, but a resilient schedule is better. It provides for a more reliable plan that is less susceptible to risk and more able to recover from unexpected external events.
Most inexperienced sea voyagers plan the shortest route to their destination, but any sailor “worth his salt” knows better. The shortest way is not always the most reliable and quickest way. Proficient sailors understand that longer routes distance-wise that have known beneficial and continuous trade-winds prove to be more reliable and less risky. Intuitively one thinks that shortening the critical path is the major duty of the project manager. But making the schedule flexible to uncertainties and unexpected events is a more productive and important effort. And this may require a focus on sub-critical paths.
This article explains why resilience is a more important schedule quality than critical path optimization.
Schedule Statistical Analysis (SSA) inspects activity duration variations and the Monte Carlo simulation examines the impact of these activity duration variations on the project schedule outcome. SSA provides a wealth of analysis on each activity including start, end, float, correlation to project duration, and probability of becoming critical.
The last parameter, the probability of a sub-critical activity becoming critical, is the most important to understand. This describes how robust (or strong) the critical path of the deterministic schedule really is. A fragile critical path is one that may be easily overtaken by other (sub-critical) paths that have a high probability of becoming critical.
A robust critical path makes for a more stable schedule. The ideal is for critical activities to remain critical, so that the critical path does not shift from one pathway through the schedule to another. Most schedulers intuitively focus on shortening the critical path, but this increases the risk of delays and probability of other sub-critical pathways becoming the new critical pathway.
It is best to have one clear definitive critical pathway through the schedule that does not compete with other sub-critical paths for dominance or importance. So a schedule that has many event chains competing for criticality is not good.
Let’s demonstrate. In Figure 1 we have a Primavera P6 Professional schedule that has three paths.
The total float of the sub-critical paths is 3-days, which is good. Observe the total float listed in the total float column and displayed on the Gantt chart (blue bars). Now we optimize the critical path in Figure 2 to compress the project duration.
As planned, the duration of the project has shortened. But in optimizing the critical path, by reducing the duration estimate of activity D, the total float of the sub-critical paths has reduced to 1-day, not good. A 2-day delay in activities B, C, E, or F would cause the critical path to shift. So in our situation, we have two sub-critical paths competing for dominance with the critical path. A moving or unstable critical path breeds confusion. Of course, our schedule is a small project and a much simplified example. However, if you are working on a large complex project, an unstable and shifting critical path could render your expensive critical path analysis efforts wasted, as the critical path has changed.
So a schedule that is robust or has a very clear critical path makes for a more resilient schedule. This supports project management decision-making and focus, which both benefit from a stable critical path. Thus, the decision-making process is made more efficient and the schedule less vulnerable to negative surprises.
Resource allocation also profits from a robust critical path or from deep sub-critical paths in relation to the critical path. Completion of subcritical activities allows for redeployment of resources to critical path activities. This is particularly true when individual resource assignments spread across multiple parallel activities. And this redeployment to crash the critical path helps compensate for unexpected delay events and makes for a more flexible schedule.
In Figure 3 we revisit our optimized schedule and start activities E and F earlier.
The total float of activities B, C, E, and F at a minimum now becomes 3-days. Great! This is what we want: an optimized and resilient schedule. We have both shortened the schedule and maintained the possibility of completing on time. Further, we have less strain on our resources. And sub-critical path resources may participate in crashing the critical path, provided they complete their activity assignments. So commencing sub-critical activities earlier may improve reliability on optimized schedules.
Another possible remedy for an uncertain sub-critical path is to provide for two or more solutions. In some situations, e.g. you order twice similar critical parts from two different suppliers thus reducing risk of missed scheduled delivery dates. This costs more and may appear as an excessive waste, but you may consider this extra expense the price of insurance. It may also be a fair investment to pay for reduced risk and increased agility.
Schedule optimization is a noble effort, but you also want your schedule to be resilient. Yes, maneuverability is an important quality of sound schedules. One way keep a schedule short and improve agility is to start sub-critical paths earlier. Also consider multiple competing solutions to boost reliability, e.g. two different suppliers providing the same critical part to ensure on time arrival.
Many are the advantages of a flexible schedule. Most notable is that it provides the project management team focus. A stable critical path will not change except for major event disruptions. And this helps support good decision-making throughout the life of the project, which is important for project success.
The first instinct of most inexperienced project managers is to optimize the schedule, but this may not produce a reliable and low-risk schedule. Schedule maneuverability helps guarantee on time delivery of deliverables and helps projects respond well to unexpected negative events. So a project manager’s efforts should focus less on schedule compression and more on improving schedule resilience.
For additional information on schedule resilience, consider reading “Advanced Scheduling Handbook for Project Managers” by Jeremie Averous and Thierry Linares.