Schedule Risk and Path Convergence
Does your schedule include an activity that has numerous predecessors? If so, your schedule may require rescheduling to avoid excessive path convergence.
An investigation of project risk should include a study on major path convergence junctures in the schedule. Convergence points are a source of significant schedule risk. They are points in the schedule where two or more activities come together and define the dependencies of a successor activity. They determine the probability of successful completion of the successor activity.
The decisive factor is the practice to stop work whenever one preceding activity is incomplete. If each predecessor comes with a probability of not completing on time, several predecessors will multiply this probability and magnify the risk of not completing all activity predecessor dependencies.
This article briefly discusses schedule path convergence, a source of major project schedule risk.
Whenever you have multiple tasks joining together at a juncture and forming the list of dependencies for a successor activity you will have increased schedule risk. These points where parallel paths merge may cause the schedule to lengthen. Typical merge spots include design reviews, start or finish of phases, and delivery dates. Any merging path can determine the merge event timing.
If two paths that have a 50% probability of completing on time merge the probability of an on time merge event is multiplicative or 25% in this situation. So risk at merge points is multiplicative. And each additional predecessor dependency represents an additional failure mode.
Paths are not necessarily a concern individually, but the completion of the merged successor is the concern. An additional path having a 50% probability of completing on time would constitute a merged successor probability of 12.5%, which is not good. As the number of predecessors increases the probability of a successful successor quickly diminishes to zero.
Predecessors that have significant total float may indicate improper sequencing of activities. Paths often converge at major milestones to simply “tie off” predecessor activities, which may not have an adequate relationship to the milestone. Review activities relationships and confirm the relevancy of predecessors.
Summary
It’s the multiplicative probability nature of merge spots that makes them particularly risky events in the life of the project. Risk spikes at convergence points. Activities that have many predecessors should be inspected to one verify these predecessor’s importance and two to consider alternative logic to link some of the respective predecessor activities to other tasks.