Path Convergence
Joining several parallel activities with a single successor activity is known as path convergence. Path convergence can be unrealistic in a plan because it implies the need to accomplish a large number of activities on time before a major event can occur as planned. The convergence of many parallel activities into a single successor—also known as a “merge point”—causes problems in managing the schedule.14
These points should be a key program management concern because risk at the merge point is multiplicative. That is, because each predecessor activity has a probability of finishing by a particular date, as the number of predecessor activities increases, the probability that the successor activity will start on time quickly diminishes to zero. Path convergence is the basis of “merge bias,” which we discuss in detail in Best Practice 8.
Because of this risk effect, activities with a great many predecessors should be examined to see if they are needed and if alternative logic can be used to link some predecessors to other activities. Predecessor activities should also be examined for available float. If many of the predecessors leading to the merge point have large amounts of float available, then convergence may not be an immediate issue. However, if several predecessor activities are determining the date of the successor event, then the workflow plan should be reexamined for the realism of performing many activities in parallel with the available resources.
Predecessors with large amounts of total float that lead to a merge point may indicate that activities are not sequenced correctly or optimally. Often paths converge because major milestones are used to “tie off” many predecessor activities, some of which may be only marginally related to the actual milestone.
The appropriate number of converging activities varies by project. The reasonable number of parallel activities is not the same in large and small projects. Because most work is performed serially, the majority of the schedule activities should have F-S relationships, in a waterfall approach to the work. An excessive number of parallel relationships can indicate an overly aggressive or unrealistic schedule.
Parallel lines do not converge in mathematics. However, in scheduling parlance, paths and activities are parallel if they occur in the same work periods. The paths converge when their activities have the same successor.↩︎