Properly placed milestones can guide you through the schedule. But improper milestones may misrepresent the reality of your true schedule situation.
Milestones are major markers in the schedule. They either tell you where you’ve been or where you are going. Milestones help to highlight the completion of a phase of work, submittal of deliverables, or project completion. They are not well suited, however, to marking the delivery of material or components from a third party vendor/supplier. This is due to the limited details that milestones provide.
This article provides a quick tip that explains why modeling supplier delivery dates as milestones (improper milestones) is not a good idea.
It is common practice for schedulers to mark supplier delivery dates with milestones. The milestone is inserted and fixed in place with a ‘start on’ constraint on the promised delivery date. So the milestone marks the date of material or component delivery. This may appear harmless, but applying a constrained milestone to mark a delivery date indicates more certainty than justified by reality. The milestone in this application misrepresents the true schedule situation.
The reality is that the milestone says little about the required supplier effort to deliver on the promised date. It is also atypical for suppliers to provide regular updates on their progress. And the risk of missing a delivery date is generally not quantified. Yet we inherently know that delivery dates are not reliable. They are risky at best.
So it is helpful to perform a risk analysis on the deliverables, but it is not possible to put a risk range on a milestone in most scheduling software. A better approach is to model the supplier effort from the time the order is placed to the promised delivery date as an actual activity.
In Figure 1 we have a schedule that includes a delivery date milestone, ‘delivery adapters’.
Figure 1
This ‘delivery adapters’ milestone is not very helpful. It provides limited information about the supplier’s effort. It does not include the duration of the supplier effort; just the date. As it does not include a duration it also not possible to examine the duration uncertainty, and provide the risk of missing the delivery date.
The preferred approach is displayed in Figure 2.
Figure 2
In this figure the ‘order and delivery adapters’ effort is indicated with a task dependent activity. The ‘order and delivery adapters’ activity would come after the pipe inspection. You have to inspect the pipe first before ordering parts to repair the piping system. The start date of this activity can be the letting of the contract date. The duration of the activity should extend to the delivery date.
The ‘order and delivery adapters’ activity is reviewable and quantifiable with a probability risk distribution. And in this way the scheduler can specify the supplier effort. It is also possible to gage the current activity and/or delivery situation by contacting the supplier and getting their current status and confidence level in the delivery date.
Summary
Milestones have their place in the echelon of scheduling tools. But milestones are not always suitable markers. Marking material delivery dates from third party vendors is a common misstep. It is better to model this effort as an activity. This allows the scheduler to perform a risk analysis on the delivery. And provides more insight into the vendor’s effort and progress.