Some schedule guidelines specify that all constraints must be contractually defined, e.g. a contract completion date. This significantly limits the application of constraints in a schedule. But what does a scheduler do when the project manager requests say a specific mobilization date (generally an important non-contractual date in the life of a project)?
Most scheduling guidelines do not absolutely forbid the use of task constraints, but they generally discourage their wide usage. Some guidelines say, again, for instance that all constraints must be contractually defined. As mentioned previously, this significantly limits the use of constraints in a schedule. The problem with constraints is twofold:
- You potentially can lose your continuous critical or longest path.
- They make the schedule static. Static schedules, i.e. schedules with many task constraints, will not respond well to changes in the project situation.
The goal is for a dynamic schedule that automatically updates task start and finish dates based on changes in task duration estimates and task relationships. What we want to consider today is whether there is a work around whereby say the mobilization date is set, as per the project manager’s request, but application of a constraint is avoided.
This article examines how to implement a specified non-contractual task start date without the insertion of a task constraint. Note schedules in this blog were created in Primavera P6 Professional.
Sometimes it appears that scheduling guidelines hamper the true narrative of the schedule situation. Why forbid or limit constraint usage, if a constraint may help to express the true description of schedule events? But as we’ve seen above, constraints have their negatives:
- Your critical or longest path can become disjointed.
- Parts of your schedule can become static.
It is therefore preferable to avoid the insertion of constraints, if at all possible. We therefore want to examine work arounds for situations we may be tempted to resort to a task constraint.
We have in Figure 1 our demonstration project.
This is a typical schedule for a government contract. Note, in particular, the gap between the ‘projected completion date’ of 10-April-2017 and the ‘contract completion date’ of 22-May-2017. Because of this buffer time between the ‘projected completion date’ and the ‘contract completion date’ the project manager can delay construction initiation or ‘mobilize equipment’ a few days and not negatively impact the completion of the project. Currently, ‘mobilize equipment’ starts on 07-February-2017. The project manager would like to delay ‘mobilize equipment’ until 20-February-2017.
So we have construction or ‘mobilize equipment’ currently set to commence on 07-February-2017, but the project manager requests a ‘mobilize equipment’ start date of 20-February-2017. The issue at hand to debate is how to define the ‘mobilize equipment’ start date of 20-February-2017. The natural logic of the schedule starts mobilization on 07-February-2017, but we need to delay mobilization until 20-February-2017. This is easily achieved by applying a ‘Start On or After’ constraint on ‘mobilize equipment’, Figure 2.
However, inserting this constraint incurs our two constraint issues:
- You lose your continuous critical or longest path.
- Your schedule becomes static.
Not good! As Figure 2 demonstrates we lose our continuous longest path when inserting a ‘Start On or After’ constraint on ‘mobilize equipment’.
What do we do? Do we force the project manager to commence ‘mobilize equipment’ earlier than desired? Or is there an alternative work around? There is a recommended work around with less inherent issues. The work around is to extend the length of ‘Gov. Review/Approval Submittals’ until ‘mobilize equipment’ starts on 20-February-2017, Figure 3.
So change the original duration of ‘Gov. Review/Approval Submittals’ from 10-days to 19-days. Purists may complain that the government only needs 10-days to review submittals. The reality is that the government can take 19-days to review submittals without negatively impacting the schedule. Whether you inform the government they have additional time or not is your discretion.
This work around only becomes an issue if you are updating schedule progress on a weekly basis. If so then ‘Gov. Review/Approval Submittals’ may finish earlier than scheduled and the question becomes why don’t you start ‘mobilize equipment’? The positives, however, are:
- You keep your continuous longest path.
- Your schedule remains dynamic.
Scheduling becomes a delicate situation of tradeoffs when you are requested to specify a task start date that does not coincide with the natural logic of the schedule. In these situations schedulers may be tempted to insert a task constraint to restrain the schedule to a desired task start date.
But broad usage of constraints is generally discouraged by scheduling guidelines. It is also preferable to find another approach in order to maintain a continuous longest path and a dynamic schedule. The compromise, however, may be as simple as extending the original duration of a predecessor activity. Shorter period progress snapshots, perhaps weekly, may have issues narrating the schedule progress when the extended task completes earlier than scheduled.
The positives of this tradeoff, however, are compelling. We must say for the record that if we had modified the finish-to-start (FS) relationship between ‘pre-construction meeting’ and ‘mobilize equipment’ by adding an amount of lag to fill the time gap we would have been able to maintain our longest path and have the ‘mobilize equipment’ constraint. However, guidelines typically say FS relationships must all be assigned 0 lag, so your use of lag is restricted.
You may also be interested in our article Scheduling Best Practice Conundrums.