We’ve recently had a few situations where folks have asked us about Schedule Margin: what is it, how does it work and when do you use it? In short, Schedule Margin is a technique for managing schedule contingency. Schedule analysts use it to help ensure that either a certain milestone or an overall schedule’s completion is protected. The end objective can be accomplished in a number of different ways, some examples of these are listed below:
- One approach is the adding of additional lag times (e.g. a lag of 3 days is increased to 5 days) between tasks to provide excess “cushion” in the hope that the task will be completed earlier than scheduled completion date. In this case additional float is incorporated into the schedule to give the operating team a higher probability of success.
- Another approach would be the addition of a “synthetic” task inserted immediately prior to the milestone task being protected. This accomplishes the same objective as item 1. However, unlike item 1, that might necessitate increasing a number of tasks lags, in this case a single such “synthetic” task’s insertion would be used to accomplish the objective of providing some level of insurance that the milestone would be accomplished on schedule.
- Another approach that is occasionally used, is to increase the duration of tasks immediately preceding the significant milestone. This gives the schedule analyst a way to ensure that the task will be completed on schedule.
- In some cases there may even be a combination of the approaches described above. This may be based upon the degree of risk that the schedule analyst and the Program Management teams have assessed for a particular milestone event.
In all cases, the objective is the same. To increase the probability of a significant milestone event being completed on schedule based upon internal and ultimately Customer needs.
Schedule Margin In Action
Whichever option or combination of options you choose to use in the establishment and control of schedule margin, the value of the float or the duration of the activity is equivalent to the schedule margin resulting from a Schedule Risk Analysis (SRA).
The following example, using Deltek Open Plan, is representative of using schedule margin float value before the deliverable, but it could just as easily been an activity.
The Project Manager has defined a schedule and Critical Path Method (CPM), is predicting a completion date of 9/27 and there is a placeholder for Schedule Margin with no duration. The Schedule Margin task (ID: 7) has no duration and there are no targets imposed on the project, so there is no impact on the Critical Path.
The Project Manager has selected a confidence factor to be used to compute the margin based on Schedule Risk Analysis. A baseline is used to capture and display the initial margin. Schedule Risk analysis predicts we have a 95% chance of completion by 10/2. In this case, we will commit to 10/2 in our contract with our customer.
The Red Arrows show the date that we are 95% confident of finishing the various activities.
The Green Schedule Margin bar is drawn between the finish predicted by CPM and the 95% confidence date selected by the Project Manager as the contractual delivery date.
After Status Update
The project has now started and progress has not gone according to plan. The slippage is now highlighted as consumption of the margin. You can see that activity 2 has not gone as planned and is now on the critical path. The impact of activity 2 running long has pushed out activities 5 and 6 and is starting to consume the original margin. The project delivery date is still protected and unchanged at 10/2.
Standard CPM Report
The standard CPM reports still work as normal and show the slippage of the project forecast completion to 9/28.
Whichever option you choose, the critical path schedules will be correct up to either the float or the activity you’ve introduced for schedule margin. As the program is executed, any changes to either the schedule margin float or activity must be documented by Baseline Change documentation approved by the Program Change Control Board. In this manner, both internal and Customer Management are cognizant of the consumption of the Program’s Schedule Margin and the reasons why it has happened.